2004/id/draft-ietf-iiir-html-01.txt

Hypertext Markup Language (HTML)          Tim Berners-Lee, CERN
Internet Draft                          Daniel Connolly, Atrium
IIIR Working Group                                    June 1993


                  Hypertext Markup Language (HTML)
                
   A Representation of Textual Information and MetaInformation
                   for Retrieval and Interchange


Status of this Document

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups.  Note that other groups may also distribute
   working documents as Internet Drafts.
   
   Internet Drafts are working documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or  obsoleted by
   other documents at any time.  It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a
   "working draft" or "work in progress".
   
   Distribution of this document is unlimited.   The document is a
   draft form of a standard for interchange of information on the
   network which is proposed  to be registered as a MIME (RFC1341)
   content type. Please send comments to timbl@info.cern.ch or the
   discussion list www-talk@info.cern.ch.
   
   This is version 1.2 of this draft. This document is available in
   hypertext on the World-Wide Web as
   http://info.cern.ch/hypertext/WWW/MarkUp/HTML.html
   
Abstract

   HyperText Markup Language (HTML) can be used to represent
   
      Hypertext news, mail, online documentation, and collaborative
      hypermedia;
      
      Menus of  options;
      
      Database query results;
      
      Simple structured documents with inlined graphics.
      
      Hypertext views of existing bodies of information
      
   The World Wide Web (W3) initiative links related information
   throughout the globe.  HTML provides one simple format for
   providing linked information, and  all W3 compatible programs are
   required to be capable of handling HTML.    W3 uses an Internet


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   protocol (Hypertext Transfer Protocol, HTTP), which allows transfer
   representations to be negotiated between client and server, the
   result being returned in an extended MIME message.  HTML is
   therefore just one, but an important one, of the representations
   used with W3.
   
   HTML is proposed as a  MIME content type.
   
   HTML refers to the URL specification of RFCxxxx.
   
   Implementations of HTML parsers and generators can be found in the
   various W3 servers and browsers, in the public domain W3 code, and
   may also be built using various public domain SGML parsers such as
   [SGMLS] .  HTML is an SGML document type with fairly generic
   semantics appropriate for representing information from a wide
   range of applications.  It is more generic than many specific SGML
   applications, but is still completely device-independent.
   
                           IN THIS DOCUMENT
                                   
   This document contains the following parts:
   
  Vocabulary             used in this document, degrees of imperative.
                         
  HTML and MIME          with discussion of character sets.
                         
  HTML and SGML          and the relationship between them,  and
                         Structured text : an introduction for
                         beginners to SGML.
                         
  HTML Elements           A list with description, example, and
                         typical rendering.
                         
  HTML Entities           Entities used to describe characters.
                         
  The HTML DTD            The text of the SGML DTD for HTML
                         
  Link relationship values .
                          A provisional list. Not part of the
                         standard.
                         
  Registration Authority
                          The authority for extending lists of valid
                         vales.
                         
  References              to related documents
                         
  Authors addresses       Contact information.
                         
   table of contents
   
Vocabulary


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   This specification uses the words below with the precise meaning
   given.
   
  Representation          The encoding of information for interchange.
                         For example, HTML is a representation of
                         hypertext.
                         
  Rendering               The form of presentation to information to
                         the human reader.
                         
  IMPERATIVES
  
  may                     The implementation is not obliged to follow
                         this in  any way.
                         
  must                    If this is not followed, the implementation
                         does not conform to this specification.
                         
  shall                   as "must"
                         
  should                  If this is not followed, though the
                         implementation officially conforms to the
                         standard, undesirable results may occur in
                         practice.
                         
  typical                 Typical rendering is described for many
                         elements. This is not a mandatory part of the
                         standard but is given as guidance for
                         designers and to help explain the uses for
                         which the elements were intended.
                         
  NOTES
  
   Sections marked "Note:" are not mandatory parts of the
   specification but for guidance only.
   
  STATUS OF FEATURES
  
  Mainstream              All parsers must recognize these features.
                         Features are mainstream unless otherwise
                         mentioned.
                         
  Extra                   Standard HTML features which may safely be
                         ignored by parsers. It is legal to ignore
                         these, treat the contents as though the tags
                         were not there. (e.g. EM, and any undefined
                         elements)
                         
  Obsolete                Not standard HTML.  Parsers should implement
                         these features as far as possible in order to
                         preserve back-compatibility with previous
                         versions of this specification.
                         

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                            HTML AND MIME
                                   
   The definition of the HTML content subtype is
   
  MIME Type name          text
                         
  MIME subtype name:      html
                         
  Required parameters:    none
                         
  Optional parameters:    charset
                         
Character sets

   The base character set (the SGML BASESET) for HTML is ISO Latin-1.
   This is the set referred to by any numeric character references .
   The actual character set used in the representation of an HTML
   document may be ISO Latin 1, or its 7-bit subset which is ASCII.
   There is no obligation for an HTML document to contain any
   characters above decimal 127.   It is possible that  a transport
   medium such as electronic mail imposes constraints on the number of
   bits in a representation of a document, though the HTTP access
   protocol used by W3 always allows 8 bit transfer.
   
   When an HTML document is encoded using 7-bit characters, then the
   mechanisms of character references and entity references may be
   used to encode characters in the upper half of the ISO Latin-1 set.
    In this way, documents may be prepared which are suitable for
   mailing through 7-bit limited systems.
   
                             INTRODUCTION
                                   
   The HyperText Markup Language is defined in terms of the ISO
   Standard Generalized Markup Language []. SGML is a system for
   defining structured document types and markup languages to
   represent instances of those document types.
   
   Every SGML document has three parts:
   
       An SGML declaration, which binds SGML processing quantities and
      syntax token names to specific values. For example, the SGML
      declaration in the HTML DTD specifies that the string that opens
      a tag is </ and the maximum length of a name is 40 characters.
      
      A prologue including one or more document type declarations,
      which specifiy the element types, element relationships and
      attributes, and references that can be represented by markup.
      The HTML DTD specifies, for example, that the HEAD element
      contains at most one TITLE element.
      
      An instance, which contains the data and markup of the document.
      
   We use the term HTML to mean both the document type and the markup


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   language for representing instances of that document type.
   
   All HTML documents share the same SGML declaration an prologue.
   Hence implementations of the WorldWide Web generally only transmit
   and store the instance part of an HTML document. To construct an
   SGML document entity for processing by an SGML parser, it is
   necessary to prefix the text from ``HTML DTD'' on page 10 to the
   HTML instance.
   
   Conversely, to implement an HTML parser, one need only implement
   those parts of an SGML parser that are needed to parse an instance
   after parsing the HTML DTD.
   
   
Structured Text

   An HTML instance is like a text file, except that some of the
   characters are interpreted as markup. The markup gives structure to
   the document.
   
   The instance represents a hierarchy of elements. Each element has a
   name , some attributes , and some content. Most elements are
   represented in the document as a start tag, which gives the name
   and attributes, followed by the content, followed by the end tag.
   For example:
   
                <HTML>
                 <TITLE>
                  A sample HTML instance
                 </TITLE>
                 <H1>
                  An Example of Structure
                 </H1>
                 Here's a typical paragraph.
                 <P>
                 <UL>
                  <LI>
                  Item one has an
                  <A NAME="anchor">
                   anchor
                  </A>
                  <LI>
                  Here's item two.
                 </UL>
                </HTML>

   Some elements (e.g. P, LI) are empty. They have no content. They
   show up as just a start tag.
   
   For the rest of the elements, the content is a sequence of data
   characters and nested elements.  Note that the HTML DTD in fact
   severely limits the amount of nesting which is allowed: most things


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   cannot be nested, in fact.   No elements may be recursively nested.
     Anchors and character highlighting may be put inside other
   constructs.
   
  TAGS
  
   Every element starts with a tag, and every non-empty element ends
   with a tag. Start tags are delimited by < and >, and end tags are
   delimited by </ and >.
   
    Names
    
   The element name immediately follows the tag open delimiter. Names
   consist of a letter followed by up to 33 letters, digits, periods,
   or hyphens. Names are not case sensitive.
   
    Attributes
    
   In a start tag, whitespace and attributes are allowed between the
   element name and the closing delimiter. An attribute consists of a
   name, an equal sign, and a value. Whitespace is allowed around the
   equal sign.
   
   The value is specified in a string surrounded by single quotes or a
   string surrounded by double quotes. (See: other tolerated forms @@)
   
   The string is parsed like RCDATA (see below ) to determine the
   attribute value. This allows, for example, quote characters in
   attribute values to be represented by character references.
   
   The length of an attribute value (after parsing) is limited to 1024
   characters.
   
  ELEMENT TYPES
  
   The name of a tag refers to an element type declaration in the HTML
   DTD. An element type declaration associates an element name with
   
      A list of attributes and their types and statuses
      
      A content type (one of EMPTY, CDATA, RCDATA, ELEMENT, or MIXED)
      which determines the syntax of the element's content
      
      A content model, which specifies the pattern of nested elements
      and data
      
    Empty Elements
    
   Empty elements have the keyword EMPTY in their declaration. For
   example:
   
        <!ELEMENT NEXTID - O EMPTY>
        <!ATTLIST NEXTID N NUMBER #REQUIRED>


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   This means that the following:
   
        <nextid n=''27''>

   is legal, but these others are not:
   
        <nextid>
        <nextid n=''abc''>

    Character Data
    
   The keyword CDATA indicates that the content of an element is
   character data. Character data is all the text up to the next end
   tag open delimiter-in-context. For example:
   
<!ELEMENT XMP - - CDATA>

   specifies that the following text is a legal XMP element:
   
        <xmp>Here's an example. It looks like it has
        <tags> and <!--comments-->
        in it, but it does not. Even this
        </ is data.</xmp>

   The string </ is only recognized as the opening delimiter of an end
   tag when it is ``in context,'' that is, when it is followed by a
   letter. However, as soon as the end tag open delimiter is
   recognized, it terminates the CDATA content. The following is an
   error:
   
        <xmp>There is no way to represent </end> tags
        in CDATA </xmp>

    Replaceable Character Data
    
   Elements with RCDATA content behave much like those with CDATA,
   except for character references and entity references. Elements
   declared like:
   
<!ELEMENT TITLE - - RCDATA>

   can have any sequence of characters in their content.
   
      Character References
      
   To represent a character that would otherwise be recognized as
   markup, use a character reference. The string &# signals a
   character reference when it is followed by a letter or a digit. The
   delimiter is followed by the decimal character number and a
   semicolon. For example:
   
<title>You can even represent &#60;/end> tags in RCDATA </title>


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      Entity References
      
   The HTML DTD declares entities for the less than, greater than, and
   ampersand characters and each of the ISO Latin 1 characters so that
   you can reference them by name rather than by number.
   
   The string & signals an entity reference when it is followed by a
   letter or a digit. The delimiter is followed by the entity name and
   a semicolon. For example:
   
Kurt G&ouml;del was a famous logician and mathematician.

  Note:                   To be sure that a string of characters has
                         no markup, HTML writers should represent all
                         occurrences of <, >, and & by character or
                         entity references.
                         
    Element Content
    
   Some elements have, in stead of a keyword that states the type of
   content, a content model, which tells what patterns of data and
   nested elements are allowed. If the content model of an element
   does not include the symbol #PCDATA , the content is element
   content.
   
   Whitespace in element content is considered markup and ignored. Any
   characters that are not markup, that is, data characters, are
   illegal.
   
   For example:
   
<!ELEMENT HEAD - - (TITLE? & ISINDEX? & NEXTID? & LINK*)>

   declares an element that may be used as follows:
   
<head>
 <isindex>
 <title>Head Example</title>
</head>

   But the following are illegal:
   
<head> no data allowed! </head>
<head><isindex><title>Two isindex tags</title><isindex></head>

    Mixed Content
    
   If the content model includes the symbol #PCDATA, the content of
   the element is parsed as mixed content. For example:
   
<!ELEMENT PRE - - (#PCDATA | A | B | I | U | P)+>
<!ATTLIST PRE
        WIDTH NUMBER #implied


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        >

   This says that the PRE element contains one or more A, B, I, U, or
   P elements or data characters. Here's an example of a PRE element:
   
<pre>
<b>NAME</b>
    cat -- concatenate<a href=''terms.html#file''>files</a>
<b>EXAMPLE</b>
    cat <xyz
</pre>

   The content of the above PRE element is:
   
      A B element
      
      The string ``   cat -- concatenate''
      
      An A element
      
      The string ``\n''
      
      Another B element
      
      The string ``\n   cat <xyz''
      
  COMMENTS AND OTHER MARKUP
  
   To include comments in an HTML document that will be ignored by the
   parser, surround them with <!-- and -->. After the comment
   delimiter, all text up to the next occurrence of -- is ignored.
   Hence comments cannot be nested. Whitespace is allowed between the
   closing -- and >. (But not between the opening <! and --.)
   
   For example:
   
<HEAD>
<TITLE>HTML Guide: Recommended Usage</TITLE>
<!-- $Id: HTML.txt,v 1.2 1994/04/12 23:13:42 connolly Exp $ -->
</HEAD>

   There are a few other SGML markup constructs that are deprecated or
   illegal.
   
  Delimiter               Signals...
                         
  <?                      Processing instruction. Terminated by >.
                         
  <![                     Marked section. Marked sections are
                         deprecated. See the SGML standard for
                         complete information.
                         
  <!                      Markup declaration. HTML defines no short


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                         reference maps, so these are errors.
                         Terminated by >.
                         
  LINE BREAKS
  
   A line break character is considered markup (and ignored) if it is
   the first or last piece of content in an element. This allows you
   to write either
   
<PRE>some example text</pre>

   or
   
<pre>
some example text
</pre>

   and these will be processed identically.
   
   Also, a line that's not empty but contains no content will be
   ignored altogether. For example, the element
   
<pre>
<!-- this line is ignored, including the linebreak character -->
first line

third line<!-- the following linebreak is content: -->
fourth line<!-- this one's ignored because it's the last piece of cont
ent: -->
</pre>

   contains only the strings
   

 first line

 third line
 fourth line.

  SPACES AND TABS
  
   Space characters must be rendered as horizontal white space.  In
   HTML, multiple spaces should be rendered as proportionally larger
   spaces.
   
   The rendering of a horizontal tab (HT) character is not defined,
   and HT should therefore not be used, except within a PRE (or
   obsolete XMP, LISTING or PLAINTEXT) element.
   
   Neither spaces nor tabs should be used to make SGML source layout
   more attractive or easier to read.
   
  SUMMARY OF MARKUP SIGNALS


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   The following delimiters may signal markup, depending on context.
   
  Delimiter               Signals
                         
  <!--                    Comment
                         
  &#                      Character reference
                         
  &                       Entity reference
                         
  </                      End tag
                         
  <!                      Markup declaration
                         
  ]]>                     Marked section close (an error)
                         
  <                       Start tag
                         
                            HTML ELEMENTS
                                   
   This is a list of elements used in the HTML language.  Documents
   should (but need not absolutely) contain an initial HEAD element
   followed by a BODY element.
   
    Old style documents may contain a just the contents of the normal
   HEAD and BODY elements, in any order. This is deprecated but must
   be supported by parsers.
   
   See also:  Status of elements
   
Properties of the whole document

   Properties of the whole document are defined by the following
   elements. They should appear within the HEAD element.  Their order
   is not significant.
   
  TITLE                   The title of the document
                         
  ISINDEX                 Sent by a server in a searchable document
                         
  NEXTID                  A parameter used by editors to generate
                         unique identifiers
                         
  LINK                    Relationship between this document and
                         another. See also the Anchor element ,
                         Relationships .  A document may have many
                         LINK elements.
                         
  BASE                    A record of the URL of the document when
                         saved
                         
Text formatting


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   These are elements which occur within the BODY element of a
   document. Their order is the logical order in which the elements
   should be rendered on the output device.
   
  Headings                Several levels of heading are supported.
                         
  Anchors                 Sections of text which form the beginning
                         and/or end of hypertext links are called
                         "anchors" and defined by the A tag.
                         
  Paragraph marks         The P element marks the break between two
                         paragraphs.
                         
  Address style           An ADDRESS element is displayed in a
                         particular style.
                         
  Blockquote style        A block of text quoted from another source.
                         
  Lists                   Bulleted lists, glossaries, etc.
                         
  Preformatted text       Sections in fixed-width font for
                         preformatted text.
                         
  Character highlighting
                          Formatting elements which do not cause
                         paragraph breaks.
                         
Graphics

  IMG                     The IMG tag allows inline graphics.
                         
Obsolete elements

   The other elements are obsolete but should be recognised by parsers
   for back-compatibility.
   
HEAD

   The HEAD element contains all information about the document in
   general.  It does not contain any text which is part of the
   document: this is in the BODY. Within the head element, only
   certain elements are allowed.
   
BODY

   The BODY element contains all the information which is part of the
   document, as opposed information about the document which is in the
   HEAD .
   
   The elements within the BODY element are in the order in which they
   should be presented to the reader.
   
   See the list of things which are allowed within a BODY element .


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Anchors

   An anchor is a piece of text which marks the beginning and/or the
   end of a hypertext link.
   
   The text between the opening tag and the closing tag is either the
   start or destination (or both) of a link. Attributes of the anchor
   tag are as follows.
   
  HREF                    OPTIONAL. If the HREF attribute is present,
                         the anchor is sensitive text: the start of a
                         link. If the reader selects this text,  (s)he
                         should be presented with another document
                         whose network address is defined by the value
                         of the HREF attribute . The format of the
                         network address is specified elsewhere . This
                         allows for the form HREF="#identifier" to
                         refer to another anchor in the same document.
                         If the anchor is in another document, the
                         attribute is a relative name , relative to
                         the documents address (or specified base
                         address if any).
                         
  NAME                    OPTIONAL. If present, the  attribute NAME
                         allows the anchor to be the destination of a
                         link. The value of the attribute is an
                         identifier for the anchor.  Identifiers are
                         arbitrary strings  but must be unique within
                         the HTML document.   Another document can
                         then make a reference explicitly to this
                         anchor by putting the identifier after the
                         address, separated by a hash sign .
                         
  REL                     OPTIONAL. An attribute REL may give the
                         relationship (s) described by the hypertext
                         link. The value is a comma-separated list of
                         relationship values.   Values and their
                         semantics will be registered by the HTML
                         registration authority . The default
                         relationship if none other is given is void.
                         REL should not be present unless HREF is
                         present. See Relationship values , REV .
                         
  REV                     OPTIONAL. The same as REL , but the
                         semantics of the link type are in the reverse
                         direction.  A link from A to B with REL="X"
                         expresses the same relationship as a link
                         from B to A with REV="X".   An anchor may
                         have both REL and REV attributes.
                         
  URN                     OPTIONAL. If present, this specifies a
                         uniform resource number for the document. See
                         note .


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  TITLE                   OPTIONAL. This is informational only. If
                         present the value of this field should equal
                         the value of the TITLE of the document whose
                         address is given by the HREF attribute. See
                         note .
                         
  METHODS                 OPTIONAL. The value of this field is a
                         string which if present must be a comma
                         separated list of HTTP METHODS supported by
                         the object for public use.  See note .
                         
   All attributes are optional, although one of NAME and HREF is
   necessary for the anchor to be useful. See also: LINK .
   
  EXAMPLE OF USE:
  
        See <A HREF="http://info.cern.ch/">CERN</A>'s information for
        more details.

        A <A NAME=serious>serious</A> crime is one which is associated
        with imprisonment.
                        ...
        The Organization may refuse employment to anyone convicted
        of a <a href="#serious">serious</A> crime.


  NOTE : UNIVERSAL RESOURCE NUMBERS
  
   URNs are provided to allow a document to be recognized if duplicate
   copies are found.  This should save a client implementation from
   picking up a copy of something it already has.
   
   The format of URNs is under discussion (1993) by various working
   groups of the Internet Engineering Task Force.
   
  NOTE:  TITLE ATTRIBUTE OF LINKS
  
   The link may carry a TITLE attribute which should if present give
   the title of the document whose address is given by the HREF
   attribute.
   
   This is useful for at least two reasons
   
      The browser software may chose to display the title of the
      document as a preliminary to retrieving it, for example as a
      margin note or on a small box while the mouse is over the
      anchor, or during document fetch.
      
      Some documents -- mainly those which are not marked up text,
      such as graphics, plain text and also  Gopher menus, do not come
      with a title themselves, and so putting a title in the link is
      the only way to give them a title. This is how Gopher works.
      Obviously it leads to duplication of data, and so it is


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      dangerous to assume that the title attribute of the link is a
      valid and unique title for the destination document.
      
  NOTE: METHODS ATTRIBUTE OF LINKS
  
   The METHODS attributes of anchors and links are used to provide
   information about the functions which the user may perform on an
   object. These are more accurately given by the HTTP protocol when
   it is used, but it may, for similar reasons as for the TITLE
   attribute, be useful to include the information in advance in the
   link.
   
   For example, The browser may chose a different rendering as a
   function of the methods allowed (for example something which is
   searchable may get a different icon)
   
Address

   This element is for address information, signatures, authorship,
   etc, often at the top or bottom of a document.
   
  TYPICAL RENDERING
  
   Typically, an address element is italic and/or right justified or
   indented.  The address element implies a paragraph break. Paragraph
   marks within the address element do not cause extra white space to
   be inserted.
   
  EXAMPLES OF USE:
  
                <ADDRESS><A HREF="Author.html">A.N.Other</A></ADDRESS>


                <ADDRESS>
                Newsletter editor<p>
                J.R. Brown<p>
                JimquickPost News, Jumquick, CT 01234<p>
                Tel (123) 456 7890
                </ADDRESS>


BASE

   This element allows the URL of the document itself to be recorded
   in situations in which the document may be read out of context.
   URLs within the  document may be in a "partial" form relative  to
   this base address.
   
   Where the base address is not specified, the reader will use the
   URL it used to access the document to resolve any relative URLs.
   
   The one attribute is:
   

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  HREF                    the URL
                         
BLOCKQUOTE

   The BLOCKQUOTE element allows text quoted from another source to be
   rendered specially.
   
  TYPICAL RENDERING
  
   A typical rendering might be a slight extra left and right indent,
   and/or italic font.  BLOCKQUOTE causes a paragraph break, and
   typically a line or so of white space will be allowed between it
   and any text before or after it.
   
   Single-font rendition may for example put a vertical line of ">"
   characters down the left margin to indicate quotation in the
   Internet mail style.
   
  EXAMPLE
  
I think it ends
<BLOCKQUOTE>Soft you now, the fair Ophelia. Nymph, in thy orisons,
be all my sins remembered.
</BLOCKQUOTE>
but I am not sure.

Headings

   Six levels  of heading are supported. (Note that a hypertext node
   within a hypertext work tends to need less levels of  heading than
   a work whose only structure is given by the nesting of headings.)
   
   A heading element implies all the font changes, paragraph breaks
   before and after, and white space (for example) necessary to render
   the heading. Further character emphasis or paragraph marks are not
   required in HTML.
   
   H1 is the highest level of heading, and is recommended for the
   start of a hypertext node.   It is suggested that the the text of
   the first heading be suitable for a reader who is already browsing
   in related information, in contrast to the title tag which should
   identify the node in a wider context.
   
   The heading elements are
   
                <H1>, <H2>, <H3>, <H4>, <H5>, <H6>

   It is not normal practice to jump from one header to a header level
   more than one below, for example for follow an H1 with an H3.
   Although this is legal, it is discouraged, as it may produce
   strange results for example when generating other representations
   from the HTML.
   

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  EXAMPLE:
  
                <H1>This is a heading</H1>
                Here is some text
                <H2>Second level heading</H2>
                Here is some more text.

  PARSER NOTE:
  
   Parsers should not require any specific order to heading elements,
   even if the heading level increases by more than one between
   successive headings.
   
  TYPICAL RENDERING
  
  H1                      Bold very large font, centered. One or two
                         lines clear space between this and anything
                         following.  If printed on paper, start new
                         page.
                         
  H2                      Bold, large font,, flush left against left
                         margin, no indent. One or two clear lines
                         above and below.
                         
  H3                      Italic, large font, slightly indented from
                         the left margin. One or two clear lines above
                         and below.
                         
  H4                      Bold, normal font, indented more than H3.
                         One clear line above and below.
                         
  H5                      Italic, normal font, indented as H4.  One
                         clear line above.
                         
  H6                      Bold, indented same as normal text, more
                         than H5. One clear line above.
                         
   These typical values are just an indication, and it is up to the
   designer of the presentation software to define the styles.  The
   reader may have options to customize these.  When writing
   documents, you should assume that whatever is done it is designed
   to have the same sort of effect as the styles above.
   
   The rendering software is responsible for generating suitable
   vertical white space between elements, so it is NOT normal or
   required to follow a heading element with a paragraph mark.
   
IMG: Embedded Images

   Status: Extra
   
   The IMG element allows another document to be inserted inline.  The
   document is normally an icon or small graphic, etc. This element is


Berners-Lee and Connolly                                             17

   NOT intended for embedding other HTML text.
   
   Browsers which are not able to display inline images ignore IMG
   elements. Authors should note that some browsers will be able to
   display (or print) linked graphics but not inline graphics. If the
   graphic is essential, it may be wiser to make a link to it rather
   than to put it inline.  If the graphic is essentially decorative,
   then IMG is appropriate.
   
   The IMG element is empty: it has no closing tag. It has two
   attributes:
   
  SRC                     The value of this attribute is the URL of
                         the document to be embedded. Its syntax is
                         the same as that of the HREF attribute of the
                         A tag. SRC is mandatory.
                         
  ALIGN                   Take values TOP or MIDDLE or BOTTOM,
                         defining whether the tops or middles of
                         bottoms of the graphics and text should be
                         aligned vertically.
                         
  ALT                    Optional alternative text as an alternative
                         to the graphics for display in text-only
                         environments.
                         
   Note that IMG elements are allowed within anchors.
   
  EXAMPLE
  
        Warning: < IMG SRC ="triangle.gif" ALT="Warning:"> This must b
e done by a
        qualified technician.

        < A HREF="Go">< IMG SRC ="Button"> Press to start</A>


ISINDEX

   This element informs the reader that the document is an index
   document. As well as reading it, the reader may use a keyword
   search.
   
   The node may be queried with a keyword search by suffixing the node
   address with a question mark, followed by a list of keywords
   separated by plus signs. See the network address format .
   
   Note that this tag is normally generated automatically by a server.
    If it is added by hand to an HTML document, then the client  will
   assume that the server can handle a search on the document.


Berners-Lee and Connolly                                             18

   Obviously the server must have this capability for it to work:
   simply adding <ISINDEX> in the document is not enough to make
   searches happen if the server does not have a search engine!
   
   Status: standard.
   
  EXAMPLE OF USE:
  
                <ISINDEX>

LINK

   The LINK element occurs within the HEAD element of an HTML
   document. It is used to indicate a relationship between the
   document and some other object.  A document may have any number of
   LINK elements.
   
   The LINK element is empty, but takes the same attributes as the
   anchor element .
   
   Typical  uses are to indicate authorship, related indexes and
   glossaries, older or more recent versions, etc.  Links can indicate
   a static tree structure in which the document was authored by
   pointing to a "parent" and "next" and "previous" document, for
   example.
   
   Servers may also allow links to be added by those who do not have
   the right to alter the body of a document.
   
Forms of list in HTML

  GLOSSARIES
  
   A glossary (or definition list) is a list of paragraphs each of
   which has a short title alongside it. Apart from glossaries, this
   element is useful for presenting a set of named elements to the
   reader. The elements within a glossary follow are
   
  DT                      The "term", typically placed in a wide left
                         indent
                         
  DD                      The "definition", which may wrap onto many
                         lines
                         
   These elements must appear in pairs. Single occurrences of DT
   without a following DD are illegal.  The one attribute which DL can
   take is
   
  COMPACT                 suggests that a compact rendering be used,
                         because the enclosed elements are
                         individually small, or the whole glossary is
                         rather large, or both.
                         

Berners-Lee and Connolly                                             19

    Typical rendering
    
   The definition list DT, DD pairs are arranged vertically.   For
   each pair, the DT element is on the left, in a column of about a
   third of the display area, and the DD element is in the right hand
   two thirds of the display area.  The DT term is normally small
   enough to fit on one line within the left-hand column. If it is
   longer, it will either extend across the page, in which case the DD
   section is moved down to separate them, or it is wrapped onto
   successive lines of the left hand column.
   
   White space is typically left between successive DT,DD pairs unless
   the COMPACT attribute is given.  The COMPACT attribute is
   appropriate for lists which are long and/or have DT,DD pairs which
   each take only a line or two.  It is of course possible for the
   rendering software to discover these cases itself and make its own
   decisions, and this is to be encouraged.
   
   The COMPACT attribute may also reduce the width of the left-hand
   (DT) column.
   
    Examples of use
    
        <DL>
        <DT>Term the first<DD>definition paragraph is reasonably
        long but is still displayed clearly
        <DT>Term2 follows<DD>Definition of term2
        </DL>

        <DL COMPACT>
        <DT>Term<DD>definition paragraph
        <DT>Term2<DD>Definition of term2
        </DL>


  LISTS
  
   A list is a sequence of paragraphs, each of which may be preceded
   by a special mark or sequence number. The syntax is:
   

                <UL>
                <LI> list element
                <LI> another list element ...
                </UL>

   The opening list tag  may be any of UL, OL, MENU or DIR.  It must
   be immediately followed by the first list element.
   
    Typical rendering
    

Berners-Lee and Connolly                                             20

   The representation of the list is not defined here, but a bulleted
   list for unordered lists,  and a sequence of numbered paragraphs
   for an ordered list would be quite appropriate. Other possibilities
   for interactive display include embedded scrollable browse panels.
   
   List elements with typical rendering are:
   
  UL                      A list of multi-line paragraphs, typically
                         separated by some white space and/or marked
                         by bullets, etc.
                         
  OL                      As UL, but the paragraphs are typically
                         numbered in some way to indicate the order as
                         significant.
                         
  MENU                    A list of smaller paragraphs. Typically one
                         line per item, with a style more compact than
                         UL.
                         
  DIR                     A list of short elements, typically less
                         than 20 characters.  These may be arranged in
                         columns across the page, typically 24
                         character in width. If the rendering software
                         is able to optimize the column width as
                         function of the widths of individual
                         elements, so much the better.
                         
    Example of use
    
                <OL>
                <LI> When you get to the station, leave
                by the southern exit, on platform one.
                <LI>Turn left to face toward the mountain
                <LI>Walk for a mile or so until you reach the
                "Asquith Arms" then
                <LI>Wait and see...
                </OL>

                < MENU >
                <LI>The oranges should be pressed fresh
                <LI>The nuts may come from a packet
                <LI>The gin must be good quality
                </MENU>

                < DIR >
                <LI>A-H<LI>I-M
                <LI>M-R<LI>S-Z
                </DIR>


Next ID


Berners-Lee and Connolly                                             21

   This tag takes a  single attribute which is the number of the next
   document-wide numeric identifier to be allocated of the form z123.
   
   When modifying a document, old anchor ids should not be reused, as
   there may be references stored elsewhere which point to them.  This
   is read and generated by hypertext editors. Human writers of HTML
   usually use mnemonic alphabetical identifiers. Browser software may
   ignore this tag.
   
  EXAMPLE OF USE:
  
                <NEXTID N=27>


P: Paragraph mark

   The empty P element indicates a paragraph break. The exact
   rendering of this (indentation,  leading, etc) is not defined here,
   and may be a function of other tags, style sheets etc.
   
   <P> is used between two pieces of text which otherwise would be
   flowed together.
   
   You do NOT need to use <P>  to put white space around heading,
   list, address or blockquote elements which imply a paragraph break.
   It is the responsibility of the rendering software to generate that
   white space.   A paragraph mark which is preceded or followed by
   such elements which imply a paragraph break is has undefined effect
   and should be avoided.
   
  TYPICAL RENDERING
  
   Typically, <P> will generate a small vertical space (of a line or
   half a line) between the paragraphs. This is not the case
   (typically) within ADDRESS  or (ever) within PRE elements.   With
   some implementations, in normal text, <P> may generate a small
   extra left indent on the first line.
   
  EXAMPLES OF USE
  
        <h1>What to do</h1>
        This is a one paragraph.< p >This is a second.
        < P >
        This is a third.

  BAD EXAMPLE
  
        <h1><P>What not to do</h1>
        <p>I found that on my XYZ browser it looked prettier to
        me if I put some paragraph marks
        <p>
        <ul><p><li>Around lists, and
        <li>After headings.


Berners-Lee and Connolly                                             22

        </ul>
        <p>
        None of the paragraph marks in this example should
        be there.


PRE: Preformatted text

   Preformatted elements in HTML are displayed with text in a fixed
   width font, and so are suitable for text which has been formatted
   for a teletype by some existing formatting system.
   

   The optional attribute is:
   
  WIDTH                  This attribute gives the maximum number of
                         characters which will occur on a line.  It
                         allows the presentation system to select a
                         suitable font and indentation. Where the
                         WIDTH attribute is not recognized, it is
                         recommended that a width of 80 be assumed.
                         Where WIDTH is supported, it is recommended
                         that at least widths of 40, 80 and 132
                         characters be presented optimally, with other
                         widths being rounded up.
                         
   Within a PRE element,
   
      Line boundaries within the text are rendered as a move to the
      beginning of the next line, except for one immediately following
      or immediately preceding a tag.
      
       The <p> tag should not be used. If found, it should be rendered
      as a move to the beginning of the next line.
      
      Anchor elements and character highlighting elements may be used.
      
      Elements which define paragraph formatting (Headings, Address,
      etc) must not be used.
      
      The ASCII Horizontal Tab (HT) character must be interpreted as
      the smallest positive nonzero number of spaces which will leave
      the number of characters so far on the line as a multiple of 8.
      Its use is not recommended however.
      
    Example of use
    
                        <PRE WIDTH="80">
                        This is an example line
                        </PRE>


Berners-Lee and Connolly                                             23

    Note: Highlighting
    
   Within a preformatted element,  the constraint that the rendering
   must be on a fixed horizontal character pitch may limit or prevent
   the ability of the renderer to render highlighting elements
   specially.
   
    Note: Margins
    
   The above references to the "beginning of a new line" must not be
   taken as implying that the renderer is forbidden from using a
   (constant) left indent for rendering preformatted text.   The left
   indent may of course be constrained by the width required.
   
TITLE

   The title of a document is specified by the TITLE element.  The
   TITLE element should occur in the HEAD of the document.
   
   There may only be one title in any document. It should identify the
   content of the document in a fairly wide context.
   
   The title is not part of the text of the document, but is a
   property of the whole document. It may not contain anchors,
   paragraph marks, or highlighting. The title may be used to identify
   the node in a history list, to label the window displaying the
   node, etc. It is not normally displayed in the text of a document
   itself. Contrast titles with headings .  The title should ideally
   be less than 64 characters in length.  That is, many applications
   will display document titles in window titles, menus, etc where
   there is only limited room.  Whilst there is no limit on the length
   of a title (as it may be automatically generated from other data),
   information providers are warned that it may be truncated if long.
   
    Examples of use
    
   Appropriate titles might be
   
                <TITLE>Rivest and Neuman. 1989(b)</TITLE>

   or
   
                <TITLE>A Recipe for Maple Syrup Flap-Jack</TITLE>

   or
   
                <TITLE>Introduction -- AFS user's Guide</TITLE>

   Examples of inappropriate titles are those which are only
   meaningful within context,
   
                <TITLE>Introduction</TITLE>


Berners-Lee and Connolly                                             24

   or too long,
   
        <TITLE>Remarks on the Quantum-Gravity effects of "Bean
        Pole" diversification in Mononucleosis patients in Developing
        Countries under Economic Conditions Prevalent during
        the Second half of the Twentieth Century, and Related Papers:
        a Summary</TITLE>


Character highlighting

   Status: Extra
   
   These elements allow sections of text to be formatted in a
   particular way, to provide emphasis, etc.  The tags do NOT cause a
   paragraph break, and may be used on sections of text within
   paragraphs.
   
   Where not supported by implementations, like all tags, these tags
   should be ignored but the content rendered.
   
   All these tags have related closing tags, as in
   
                This is <EM>emphasized</EM> text.

   Some of these styles are more explicit than others about how they
   should be physically represented.  The logical styles should be
   used wherever possible, unless for example it is necessary to refer
   to the formatting in the text. (Eg, "The italic parts are
   mandatory".)
   
    Note:
    
   Browsers unable to display a specified style may render it in some
   alternative, or the default, style, with some loss of quality for
   the reader. Some implementations may ignore these tags altogether,
   so information providers should attempt not to rely on them as
   essential to the information content.
   
   These element names are derived from TeXInfo macro names.
   
  PHYSICAL STYLES
  
  TT                      Fixed-width typewriter font.
                         
  B                       Boldface, where available, otherwise
                         alternative mapping allowed.
                         
  I                       Italic font (or slanted if italic
                         unavailable).
                         

Berners-Lee and Connolly                                             25

  U                       Underline.
                         
  LOGICAL STYLES
  
  EM                      Emphasis, typically italic.
                         
  STRONG                  Stronger emphasis, typically bold.
                         
  CODE                    Example of code. typically monospaced font.
                         (Do not confuse with  PRE )
                         
  SAMP                    A sequence of literal characters.
                         
  KBD                     in an instruction manual, Text typed by a
                         user.
                         
  VAR                     A variable name.
                         
  DFN                     The defining instance of a term. Typically
                         bold or bold italic.
                         
  CITE                    A citation. Typically italic.
                         
  EXAMPLES OF USE
  
        This text contains an <em>emphasized</em> word.
        <strong>Don't assume</strong> that it will be italic!
        It was made using the <CODE>EM</CODE> element. A citation is
        typically italic and has no formal necessary structure:
        <cite>Moby Dick</cite> is a book title.


Obsolete elements

   The following elements of HTML are obsolete.  It is recommended
   that client implementors implement the obsolete forms for
   compatibility with old servers.
   
    Plaintext
    
   Status: Obsolete .
   
   The empty PLAINTEXT tag terminates the HTML entity. What follows is
   not SGML. In stead, there's an old HTTP convention that what
   follows is an ASCII  (MIME "text/plain") body.
   
   An example if its use is:
   
                        <PLAINTEXT>
                        0001 This is line one of a ling listing
                        0002 file from <any@host.inc.com> which is sen


Berners-Lee and Connolly                                             26

t

   This tag allows the rest of a file to be read efficiently without
   parsing. Its presence is an optimization. There is no closing tag.
   The rest of the data is not in SGML.
   
    XMP and LISTING:   Example sections
    
   Status:  Obsolete . This are in use and should be recognized by
   browsers. New servers should use <PRE> instead.
   
   These styles allow text of fixed-width characters to be embedded
   absolutely as is into the document. The syntax is:
   
                        <LISTING>
                                ...
                        </LISTING>

   or
   
                        <XMP>
                                ...
                        </XMP>

   The text between these tags is to be portrayed in a fixed width
   font, so that any formatting done by character spacing on
   successive lines will be maintained. Between the opening and
   closing tags:
   
      The text may contain any ISO Latin printable characters, but not
      the end tag opener. (See Historical note )
      
      Line boundaries are significant, except any occurring
      immediately after the opening tag or before the closing tag. and
      are to be rendered as a move to the start of a new line.
      
      The ASCII Horizontal Tab (HT) character must be interpreted as
      the smallest positive nonzero number of spaces which will leave
      the number of characters so far on the line as a multiple of 8.
      Its use is not recommended however.
      
   The LISTING element is portrayed so that at least 132 characters
   will fit on a line.  The XMP elementis portrayed in a font so that
   at least 80 characters will fit on a line but is otherwise
   identical to LISTING.
   
    Highlighted Phrase HP1 etc
    
   Status: Obsolete . These tags like all others should be ignored if
   not implemented. Replaced will more meaningful elements -- see
   character highlighting .
   
      Examples of use:


Berners-Lee and Connolly                                             27

                <HP1>...</HP1>   <HP2>... </HP2> etc.

    Comment element
    
   Status: Obsolete
   
   A comment element used for bracketing off unneed text and comment
   has been introduced in some browsers but will be replaced by the
   SGML command feature in new implementations.
   
  HISTORICAL NOTE: XMP AND LISTING
  
   The XMP and LISTING elements used historically to have non SGML
   conforming specifications, in that the text could contain any ISO
   Latin printable characters, including the tag opener, so long as it
   does not contain the closing tag in full.
   
   This form is not supported by SGML and so is not the specified HTML
   interpretation.  Providers should be warned that implementations
   may vary on how they interpret end tags apparently within these
   elements
   
                               ENTITIES
                                   
   The following entity names are used in HTML , always prefixed by
   ampersand (&) and followed by a semicolon as shown.  They represent
   particular graphic characters which have special meanings in places
   in the markup, or may not be part of the character set available to
   the writer.
   
  &lt;                    The less than sign <
                         
  &gt;                    The "greater than" sign >
                         
  &amp;                   The ampersand sign & itself.
                         
  &quot;                  The double quote sign "
                         
   Also allowed are references to any of the ISO Latin-1 alphabet,
   using the entity names in the following table.
   
ISO Latin 1 character entities

   This list is derived from "ISO 8879:1986//ENTITIES Added Latin
   1//EN".
   
  &AElig;                capital AE diphthong (ligature)
                         
  &Aacute;               capital A, acute accent
                         
  &Acirc;                capital A, circumflex accent
                         
  &Agrave;               capital A, grave accent


Berners-Lee and Connolly                                             28

  &Aring;                capital A, ring
                         
  &Atilde;               capital A, tilde
                         
  &Auml;                 capital A, dieresis or umlaut mark
                         
  &Ccedil;               capital C, cedilla
                         
  &ETH;                  capital Eth, Icelandic
                         
  &Eacute;               capital E, acute accent
                         
  &Ecirc;                capital E, circumflex accent
                         
  &Egrave;               capital E, grave accent
                         
  &Euml;                 capital E, dieresis or umlaut mark
                         
  &Iacute;               capital I, acute accent
                         
  &Icirc;                capital I, circumflex accent
                         
  &Igrave;               capital I, grave accent
                         
  &Iuml;                 capital I, dieresis or umlaut mark
                         
  &Ntilde;               capital N, tilde
                         
  &Oacute;               capital O, acute accent
                         
  &Ocirc;                capital O, circumflex accent
                         
  &Ograve;               capital O, grave accent
                         
  &Oslash;               capital O, slash
                         
  &Otilde;               capital O, tilde
                         
  &Ouml;                 capital O, dieresis or umlaut mark
                         
  &THORN;                capital THORN, Icelandic
                         
  &Uacute;               capital U, acute accent
                         
  &Ucirc;                capital U, circumflex accent
                         
  &Ugrave;               capital U, grave accent
                         
  &Uuml;                 capital U, dieresis or umlaut mark
                         
  &Yacute;               capital Y, acute accent
                         
  &aacute;               small a, acute accent


Berners-Lee and Connolly                                             29

  &acirc;                small a, circumflex accent
                         
  &aelig;                small ae diphthong (ligature)
                         
  &agrave;               small a, grave accent
                         
  &aring;                small a, ring
                         
  &atilde;               small a, tilde
                         
  &auml;                 small a, dieresis or umlaut mark
                         
  &ccedil;               small c, cedilla
                         
  &eacute;               small e, acute accent
                         
  &ecirc;                small e, circumflex accent
                         
  &egrave;               small e, grave accent
                         
  &eth;                  small eth, Icelandic
                         
  &euml;                 small e, dieresis or umlaut mark
                         
  &iacute;               small i, acute accent
                         
  &icirc;                small i, circumflex accent
                         
  &igrave;               small i, grave accent
                         
  &iuml;                 small i, dieresis or umlaut mark
                         
  &ntilde;               small n, tilde
                         
  &oacute;               small o, acute accent
                         
  &ocirc;                small o, circumflex accent
                         
  &ograve;               small o, grave accent
                         
  &oslash;               small o, slash
                         
  &otilde;               small o, tilde
                         
  &ouml;                 small o, dieresis or umlaut mark
                         
  &szlig;                small sharp s, German (sz ligature)
                         
  &thorn;                small thorn, Icelandic
                         
  &uacute;               small u, acute accent
                         
  &ucirc;                small u, circumflex accent


Berners-Lee and Connolly                                             30

  &ugrave;               small u, grave accent
                         
  &uuml;                 small u, dieresis or umlaut mark
                         
  &yacute;               small y, acute accent
                         
  &yuml;                 small y, dieresis or umlaut mark
                         
                             THE HTML DTD
                                   
   The HTML DTD follows . Its relationship to the content of an SGML
   document is explained in the section "HTML and SGML" .
   

<!SGML  "ISO 8879:1986"
--
        Document Type Definition for the HyperText Markup Language
        as used by the World Wide Web application (HTML DTD).

        NOTE: This is a definition of HTML with respect to
        SGML, and assumes an understanding of SGML terms.
--

CHARSET
         BASESET  "ISO 646:1983//CHARSET
                   International Reference Version (IRV)//ESC 2/5 4/0"
         DESCSET  0   9   UNUSED
                  9   2   9
                  11  2   UNUSED
                  13  1   13
                  14  18  UNUSED
                  32  95  32
                  127 1   UNUSED
     BASESET   "ISO Registration Number 100//CHARSET
                ECMA-94 Right Part of Latin Alphabet Nr. 1//ESC 2/13 4
/1"
     DESCSET   128 32 UNUSED
               160 95 32
               255  1 UNUSED


CAPACITY        SGMLREF
                TOTALCAP        150000
                GRPCAP          150000

SCOPE    DOCUMENT
SYNTAX
         SHUNCHAR CONTROLS 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
 18
                           19 20 21 22 23 24 25 26 27 28 29 30 31 127
255
         BASESET  "ISO 646:1983//CHARSET
                   International Reference Version (IRV)//ESC 2/5 4/0"


Berners-Lee and Connolly                                             31

         DESCSET  0 128 0
         FUNCTION RE          13
                  RS          10
                  SPACE       32
                  TAB SEPCHAR  9
         NAMING   LCNMSTRT ""
                  UCNMSTRT ""
                  LCNMCHAR ".-"
                  UCNMCHAR ".-"
                  NAMECASE GENERAL YES
                           ENTITY  NO
         DELIM    GENERAL  SGMLREF
                  SHORTREF SGMLREF
         NAMES    SGMLREF
         QUANTITY SGMLREF
                  NAMELEN  34
                  TAGLVL   100
                  LITLEN   1024
                  GRPGTCNT 150
                  GRPCNT   64

FEATURES
  MINIMIZE
    DATATAG  NO
    OMITTAG  NO
    RANK     NO
    SHORTTAG NO
  LINK
    SIMPLE   NO
    IMPLICIT NO
    EXPLICIT NO
  OTHER
    CONCUR   NO
    SUBDOC   NO
    FORMAL   YES
  APPINFO    NONE
>

<!DOCTYPE HTML [
<!-- Jul 1 93 -->
<!--    Regarding clause 6.1, SGML Document:

        [1] SGML document = SGML document entity,
            (SGML subdocument entity |
            SGML text entity | non-SGML data entity)*

        The role of SGML document entity is filled by this DTD,
        followed by the conventional HTML data stream.
-->

<!-- DTD definitions -->

<!ENTITY % heading "H1|H2|H3|H4|H5|H6" >


Berners-Lee and Connolly                                             32

<!ENTITY % list "UL|OL|DIR|MENU">
<!ENTITY % literal "XMP|LISTING">

<!ENTITY % headelement
         "TITLE|NEXTID|ISINDEX" >

<!ENTITY % bodyelement
         "P | %heading |
         %list | DL | HEADERS | ADDRESS | PRE | BLOCKQUOTE
        | %literal">

<!ENTITY % oldstyle "%headelement | %bodyelement | #PCDATA">

<!ENTITY % URL "CDATA"
        -- The term URL means a CDATA attribute
           whose value is a Uniform Resource Locator,
           as defined. (A URN may also be usable here when defined.)
        -->

<!ENTITY % linkattributes
        "NAME NMTOKEN #IMPLIED
        HREF %URL; #IMPLIED
        REL CDATA #IMPLIED -- forward relationship type --
        REV CDATA #IMPLIED -- reversed relationship type
                              to referent data:

                                PARENT CHILD, SIBLING, NEXT, TOP,
                                DEFINITION, UPDATE, ORIGINAL etc. --

        URN CDATA #IMPLIED -- universal resource number --

        TITLE CDATA #IMPLIED -- advisory only --

        METHODS NAMES #IMPLIED -- supported public methods of the obje
ct:
                                        TEXTSEARCH, GET, HEAD, ... --

        ">


<!-- Document Element -->

<!ELEMENT HTML O O  (( HEAD | BODY | %oldstyle)*, PLAINTEXT?)>

<!ELEMENT HEAD - -  (TITLE? & ISINDEX? & NEXTID? & LINK*
                              & BASE?)>

<!ELEMENT TITLE - -  RCDATA
          -- The TITLE element is not considered part of the flow of t
ext.
             It should be displayed, for example as the page header or
             window title.
          -->


Berners-Lee and Connolly                                             33

<!ELEMENT ISINDEX - O EMPTY
          -- WWW clients should offer the option to perform a search o
n
             documents containing ISINDEX.
          -->

<!ELEMENT NEXTID - O EMPTY>
<!ATTLIST NEXTID N NAME #REQUIRED
          -- The number should be a name suitable for use
             for the ID of a new element. When used, the value
             has its numeric part incremented. EG Z67 becomes Z68
          -->
<!ELEMENT LINK - O EMPTY>
<!ATTLIST LINK
        %linkattributes>
        
<!ELEMENT BASE - O EMPTY    -- Reference context for URLS -->
<!ATTLIST BASE

        HREF %URL; #IMPLIED

        >
<!ENTITY % inline "EM | TT | STRONG | B | I | U |
                        CODE | SAMP | KBD | KEY | VAR | DFN | CITE "
        >

<!ELEMENT (%inline;) - - (#PCDATA)>

<!ENTITY % text "#PCDATA | IMG | %inline;">

<!ENTITY % htext "A | %text">

<!ELEMENT BODY - -  (%bodyelement|%htext;)*>


<!ELEMENT A     - -  (%text)>
<!ATTLIST A
        %linkattributes;
        >

<!ELEMENT IMG    - O EMPTY --  Embedded image -->
<!ATTLIST IMG
        SRC %URL; #IMPLIED      -- URL of document to embed --
        >


<!ELEMENT P     - O EMPTY -- separates paragraphs -->

<!ELEMENT ( %heading )  - -  (%htext;)+>

<!ELEMENT DL    - -  (DT | DD | P | %htext;)*>
<!--    Content should match ((DT,(%htext;)+)+,(DD,(%htext;)+))
        But mixed content is messy.


Berners-Lee and Connolly                                             34

  -->

<!ELEMENT DT    - O EMPTY>
<!ELEMENT DD    - O EMPTY>

<!ELEMENT (UL|OL) - -  (%htext;|LI|P)+>
<!ELEMENT (DIR|MENU) - -  (%htext;|LI)+>
<!--    Content should match ((LI,(%htext;)+)+)
        But mixed content is messy.
  -->
<!ATTLIST (%list)
        COMPACT NAME #IMPLIED -- COMPACT, etc.--
        >

<!ELEMENT LI    - O EMPTY>

<!ELEMENT BLOCKQUOTE - - (%htext;|P)+
        -- for quoting some other source -->

<!ELEMENT ADDRESS - - (%htext;|P)+>

<!ELEMENT PRE - - (#PCDATA|%inline|A|P)+>
<!ATTLIST PRE
        WIDTH NUMBER #implied
        >

<!-- Mnemonic character entities. -->
<!ENTITY AElig "&#198;" -- capital AE diphthong (ligature) -->
<!ENTITY Aacute "&#193;" -- capital A, acute accent -->
<!ENTITY Acirc "&#194;" -- capital A, circumflex accent -->
<!ENTITY Agrave "&#192;" -- capital A, grave accent -->
<!ENTITY Aring "&#197;" -- capital A, ring -->
<!ENTITY Atilde "&#195;" -- capital A, tilde -->
<!ENTITY Auml "&#196;" -- capital A, dieresis or umlaut mark -->
<!ENTITY Ccedil "&#199;" -- capital C, cedilla -->
<!ENTITY ETH "&#208;" -- capital Eth, Icelandic -->
<!ENTITY Eacute "&#201;" -- capital E, acute accent -->
<!ENTITY Ecirc "&#202;" -- capital E, circumflex accent -->
<!ENTITY Egrave "&#200;" -- capital E, grave accent -->
<!ENTITY Euml "&#203;" -- capital E, dieresis or umlaut mark -->
<!ENTITY Iacute "&#205;" -- capital I, acute accent -->
<!ENTITY Icirc "&#206;" -- capital I, circumflex accent -->
<!ENTITY Igrave "&#204;" -- capital I, grave accent -->
<!ENTITY Iuml "&#207;" -- capital I, dieresis or umlaut mark -->
<!ENTITY Ntilde "&#209;" -- capital N, tilde -->
<!ENTITY Oacute "&#211;" -- capital O, acute accent -->
<!ENTITY Ocirc "&#212;" -- capital O, circumflex accent -->
<!ENTITY Ograve "&#210;" -- capital O, grave accent -->
<!ENTITY Oslash "&#216;" -- capital O, slash -->
<!ENTITY Otilde "&#213;" -- capital O, tilde -->
<!ENTITY Ouml "&#214;" -- capital O, dieresis or umlaut mark -->
<!ENTITY THORN "&#222;" -- capital THORN, Icelandic -->
<!ENTITY Uacute "&#218;" -- capital U, acute accent -->


Berners-Lee and Connolly                                             35

<!ENTITY Ucirc "&#219;" -- capital U, circumflex accent -->
<!ENTITY Ugrave "&#217;" -- capital U, grave accent -->
<!ENTITY Uuml "&#220;" -- capital U, dieresis or umlaut mark -->
<!ENTITY Yacute "&#221;" -- capital Y, acute accent -->
<!ENTITY aacute "&#225;" -- small a, acute accent -->
<!ENTITY acirc "&#226;" -- small a, circumflex accent -->
<!ENTITY aelig "&#230;" -- small ae diphthong (ligature) -->
<!ENTITY agrave "&#224;" -- small a, grave accent -->
<!ENTITY amp "&amp;" -- ampersand -->
<!ENTITY aring "&#229;" -- small a, ring -->
<!ENTITY atilde "&#227;" -- small a, tilde -->
<!ENTITY auml "&#228;" -- small a, dieresis or umlaut mark -->
<!ENTITY ccedil "&#231;" -- small c, cedilla -->
<!ENTITY eacute "&#233;" -- small e, acute accent -->
<!ENTITY ecirc "&#234;" -- small e, circumflex accent -->
<!ENTITY egrave "&#232;" -- small e, grave accent -->
<!ENTITY eth "&#240;" -- small eth, Icelandic -->
<!ENTITY euml "&#235;" -- small e, dieresis or umlaut mark -->
<!ENTITY gt "&#62;" -- greater than -->
<!ENTITY iacute "&#237;" -- small i, acute accent -->
<!ENTITY icirc "&#238;" -- small i, circumflex accent -->
<!ENTITY igrave "&#236;" -- small i, grave accent -->
<!ENTITY iuml "&#239;" -- small i, dieresis or umlaut mark -->
<!ENTITY lt "&lt;" -- less than -->
<!ENTITY ntilde "&#241;" -- small n, tilde -->
<!ENTITY oacute "&#243;" -- small o, acute accent -->
<!ENTITY ocirc "&#244;" -- small o, circumflex accent -->
<!ENTITY ograve "&#242;" -- small o, grave accent -->
<!ENTITY oslash "&#248;" -- small o, slash -->
<!ENTITY otilde "&#245;" -- small o, tilde -->
<!ENTITY ouml "&#246;" -- small o, dieresis or umlaut mark -->
<!ENTITY szlig "&#223;" -- small sharp s, German (sz ligature) -->
<!ENTITY thorn "&#254;" -- small thorn, Icelandic -->
<!ENTITY uacute "&#250;" -- small u, acute accent -->
<!ENTITY ucirc "&#251;" -- small u, circumflex accent -->
<!ENTITY ugrave "&#249;" -- small u, grave accent -->
<!ENTITY uuml "&#252;" -- small u, dieresis or umlaut mark -->
<!ENTITY yacute "&#253;" -- small y, acute accent -->
<!ENTITY yuml "&#255;" -- small y, dieresis or umlaut mark -->

<!-- deprecated elements -->

<!ELEMENT (%literal) - -  CDATA>

<!ELEMENT PLAINTEXT - O EMPTY>

<!-- Local Variables: -->
<!-- mode: sgml -->
<!-- compile-command: "sgmls -s -p " -->
<!-- end: -->
]>


Berners-Lee and Connolly                                             36

                       LINK RELATIONSHIP VALUES
                                   
   Status: This list is not part of the standard.  It is intended to
   illustrate the use of link relationships and to provide a framework
   for further development.
   
   Additions to this list will be controlled by the HTML registration
   authority . Experimental values may be used on the condition that
   they begin with "X-".
   
   These values of the REL attribute of hypertext links have a
   significance defined here, and may be treated in special ways by
   HTML applications.
   
   These relationships relate whole documents (objects), rather than
   particular anchors within them. If the relationship value is used
   with a link between anchors rather than whole documents, the
   semantics are considered to apply to the documents.
   
   In the explanations which follows, A is the source document of the
   link and B is the destination document specified by the HREF
   attribute.
   
   A relationship marked "Acyclic" has the property that no sequence
   of links with that relationship may be followed from any document
   back to itself. These types of links may therefore be used to
   define trees.
   
Relationships between documents

   These relationships are between the documents themselves rather
   than the subjects of the documents.
   
  USEINDEX
  
   B is a related index for a search by a user reading this document
   who asks for an index search function.
   
   A document may have any number of index links, causing several
   indexes top be searched in a client-defined manner.
   
   B must support SEARCH operations under its access protocol.
   
  USEGLOSSARY
  
   B is an index which should be used to resolve glossary queries in
   the document. (Typically, a double-click on a word which is not
   within an anchor).
   
   A document may have any number of glossary links.
   
  ANNOTATION
  

Berners-Lee and Connolly                                             37

   The information in B is additional to and subsidiary to that in A.
   
   Annotation is used by one person to write the equivalent of "margin
   notes" or other criticism on another's document, for example.
   
   Example: The relationship between a newsgroup and its articles.
   
   Acyclic.
   
  REPLY
  
   Similar to Annotation, but there is no suggestion that B is
   subsidiary to A: A and B are on equal footings.
   
   Example: The relationship between a mail message and its reply, a
   news article and its reply.
   
   Acyclic.
   
  EMBED
  
   If this link is followed, the node at the end of it is embedded
   into the display of the source document.
   
   Acyclic.
   
  PRECEDES
  
   In an ordered structure defined by the author, A precedes B, B is
   followed by A.
   
   Acyclic.
   
   Any document may only have one link of this relationship, and/or
   one link of the reverse relationship.
   
   Note: May be used to control navigational aids, generate printed
   material, etc.  In conjunction with " subdocument ", may be used to
   define a tree such as a printed book made of hypertext document.
   The document can only have one such tree.
   
  SUBDOCUMENT
  
   B is a lower part in the author's hierarchy to A.  Acyclic. See
   also Precedes .
   
  PRESENT
  
   Whenever A is presented, B must also be presented.  This implies
   that whenever A is retrieved, B must also be retrieved.
   
  SEARCH
  

Berners-Lee and Connolly                                             38

   When the link is followed, the node B should be searched rather
   than presented. That is, where the client software allows it, the
   user should immediately be presented with a search panel and
   prompted for text. The search is then performed without an
   intermediate retrieval or presentation of the node B
   
  SUPERSEDES
  
   B is a previous version of  A.
   
   Acyclic.
   
  HISTORY
  
   B is a list of versions of A
   
   A link reverse link must exist from B to A and to all other known
   versions of A.
   
Relationships about subjects of documents

   These relationships convey semantics about objects described by
   documents, rather than the documents themselves.
   
  INCLUDES
  
   A includes B, B is part of A.  For example, a person described by
   document A is a part of the group described by document B.
   
   Acyclic.
   
  MADE
  
   Person (etc) described by node A is author of, or is responsible
   for B
   
   This information can be used for protection, and informing authors
   of interest, for sending mail to authors, etc.
   
  INTERESTED
  
   Person (etc) described by A is interested in node B.
   
   This information can be used for notification of changes.
   
   Typically, this is a request that, when object B changes in some
   way, a new link is made to object A.
   
   The phrase "object B changes" may be interpreted narrowly (as "B
   itself changes") or widely (as "B or anythink linked to it or
   related to it closely changes").   The amount of  change considered
   worth notifying people about is also subject to interpretation,
   varying from bit changes in the source to a "new edition" statement


Berners-Lee and Connolly                                             39

   by the publisher.
   
                        REGISTRATION AUTHORITY
                                   
   The HTTP Registration Authority is responsible for maintaining
   lists of:
   
      Relationship names for link and anchor elements
      
   It is proposed that the Internet Assigned Numbers Authority or
   their successors take this role.
   
   Unregistered values may be used for experimental purposes if they
   are start with "X-".
   
                              REFERENCES
                                   
  SGML                    ISO 8879:1986, Information Processing Text
                         and Office Systems Standard Generalized
                         Markup Language (SGML).
                         
  sgmls                   an SGML parser by James Clark
                         <jjc@jclark.com> derived from the ARCSGML
                         parser materials which were written by
                         Charles F. Goldfarb. The source is available
                         on the ifi.uio.no FTP server in the directory
                         /pub/SGML/SGMLS .
                         
  WWW                     The World-Wide Web , a global information
                         initiative. For bootstrap information, telnet
                         info.cern.ch or find documents by
                         ftp://info.cern.ch/pub/www/doc
                         
  URL                     Universal Resource Locators. RFCxxx.
                         Currently  available by anonymous FTP from
                         info.cern.ch in /pub/ietf.
                         
                          AUTHOR'S ADDRESSES
                                   
   This document was prepared with the help and advice of many people
   across the net.  Dan Connolly prepared the DTD and the section on
   HTML and SGML whilst with Convex Computer Corporation of 3000
   Waterview Parkway Richardson, TX 75083. He is now with Atrium
   Technology Inc., and is not a current editor of the document.
   
                                Tim Berners-Lee
                Address         CERN
                                1211 Geneva 23
                                Switzerland
                Telephone:      +41(22)767 3755
                Fax:            +41(22)767 7155
                email:          timbl@info.cern.ch


Berners-Lee and Connolly                                             40

                                Daniel Connolly
                Address:        Atrium Technologies, Inc.
                                5000 Plaza on the Lake, Suite 275
                                Austin, TX 78746
                                USA
                email:          connolly@atrium.com


Berners-Lee and Connolly                                             41
