2004/id/draft-ietf-html-cda-00.txt


   INTERNET DRAFT                               Paul Burchard,  
                                                  Princeton CS
   Expires in six months                        Dave Raggett, W3  
                                                  Consortium

                          Compound Documents in HTML

                         <draft-ietf-html-cda-00.txt>

Status of this Memo

   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 information as Internet drafts. 

   Internet Drafts are draft documents valid for a maximum of six
   months and can be updated, replaced or obsoleted by other documents
   at any time. It is inappropriate to use Internet drafts as reference
   material or to cite them as other than as "work in progress". 

   To learn the current status of any Internet draft please check the
   "lid-abstracts.txt" listing contained in the Internet drafts shadow
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   munnari.oz.au (Pacific Rim), ds.internic.net (US East coast) or
   ftp.isi.edu (US West coast). Further information about the IETF can
   be found at URL: http://www.cnri.reston.va.us/ 

   Distribution of this document is unlimited. Please send comments to
   the HTML working group (HTML-WG) of the Internet Engineering Task
   Force (IETF) at <html-wg@oclc.org>. Discussions of this group are
   archived at URL:  http://www.acl.lanl.gov/HTML-WG/archives.html. 

1. Abstract

   This specification provides an HTML implementation of a simple
   compound document architecture for the World Wide Web, based on a
   new <EMBED> element. 

   By not restricting itself to a limited class of media types or media
   handler implementations, this element enables portable compound
   document markup, and encourages the modular design of user-agents.
   Although this specification does not presume to define a concrete
   API between extensible user-agents and their media handlers, some
   high-level requirements are imposed on the embedding semantics in
   order to ensure support for the full linking and embedding model. 

   By making <EMBED> a container element, rich alternative text with
   links and images is enabled. Moreover, the container element
   provides superior extensibility, setting the stage for structured
   enhancement of SGML content models, rather than sole dependence on
   proliferation of attributes. 

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Contents

   
   1.  Abstract   
.......................................................... 1

   2.  Examples and Rationale   
............................................ 2

   3.  Compound Document Architecture   
.................................... 4

   4.  Geometry Negotiation   
.............................................. 5

   5.  HTML Markup   
....................................................... 5

       a)  Elements   
...................................................... 5

       b)  Attribute Value Types   
......................................... 8

       c)  Attribute Sets   
................................................ 9

   6.  Summary of DTD Changes for HTML   
.................................. 14

   7.  Transition Issues   
................................................ 18

       a)  Sun's APPLET Tag   
............................................. 18

       b)  Netscape's Early Implementations of EMBED   
.................... 18

   8.  Security Considerations   
.......................................... 18

   9.  Acknowledgments   
.................................................. 18

   10. References   
....................................................... 19

   11. Authors' Addresses   
............................................... 19

2. Examples and Rationale

   The only compound document feature defined in HTML 2.0 [1] was the
   <IMG> tag for embedding image media into HTML. Although arguably,
   next to <A>, the single most influential tag in the explosive growth
   of the Web, its shortcomings are now causing a proliferation of
   media- and user-agent-specific attempts at extending HTML. 

   The most serious shortcoming of the <IMG> tag is its arbitrary
   restriction to image media. Many of the proposed embedding
   extensions also use names which suggest specialized functionality
   (e.g. <APPLET>, <FIG>). But as modular, extensible user agents
   become the norm, such restrictions become untenable. We propose that
   the name EMBED is both broad and intuitive enough to denote a
   generic embedding element. In order to avoid
   implementation-dependent markup, it is essential that this EMBED tag
   should cover all embeddable Internet media types [2], however the

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   corresponding media handlers are implemented (`built-ins',
   `plug-ins', etc.). 

   The second key shortcoming of the <IMG> tag is that it is `empty'
   (meaning that it doesn't use a closing </IMG>). Since SGML
   attributes are the only mechanism then available for the expression
   of element properties, all the powerful structuring capabilities of
   SGML are lost. In particular, the inability to nest elements inside
   an empty element means that properties with rich text values (such
   as would be desirable for the alternative text of <IMG>) cannot be
   implemented. 

   Moreover, extensibility of empty elements suffers because everything
   occurs in the a single, flat attribute namespace. Whereas empty
   elements with complex properties create an "attribute soup" that may
   not even be legal SGML: 

   
        <IMG APPLETSRC="my.applet" ALT="[applet]" FOO="23.8"  
BAR="5.0" ...>
        <!-- WRONG: cannot pass arbitrary attributes -->
   

   complex container elements can grow in a natural way, benefitting
   from the structured design of SGML: 

   
        <EMBED SRC="my.applet">
                <PARAM NAME="foo" VALUE="23.8">
                <PARAM NAME="bar" VALUE="5.0">
                ...
                <em>Download</em> this cool <A  
HREF="my.exe">application</A>!
                <!-- rich alternative text -->
        </EMBED>
   

   The ability to pass an extensible set of parameters to the embedded
   media handler in this way is one of the main requirements for an
   embedding tag. 

   Notice that this structured content approach also makes it possible
   to combine <EMBED> with <IMG> for excellent backwards compatibility: 

   
        <EMBED SRC="movie.mpg">
                <IMG SRC="movie.001.jpg"> <!-- first frame -->
        </EMBED>
   

   Here, an image of the first frame of the movie will be shown
   whenever the movie itself is not displayed, providing fallback for
   all of the following situations: 

   *   the user-agent does not know about the <EMBED> tag, or 

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   *   the user-agent does not have an embeddable handler for the
       video/mpeg media type, or 

   *   the linking and embedding fails in any other way. 

3. Compound Document Architecture

   Because the Web, as a distributed hypermedia system, makes a clear
   separation between storage and presentation of data, the model
   described in this section would more accurately be called a
   "compound presentation architecture". The model offered here takes a
   very high-level view of client-side state, and explains how links,
   the fundamental building blocks of the Web, are used to assemble
   compound presentations in stateful user-agents. 

   The model views the user-agent as a repository of hierarchical
   presentation resources. A presentation resource is an addressable
   unit of state in the user-agent, such as the document display area
   of a GUI window. An embedded display area (say, showing a video)
   within the larger display area (say, showing an HTML document) would
   be a child in the presentation resource hierarchy. This hierarchical
   structure makes it possible to use URLs (within the generic-RL
   paradigm [3]) as addresses, but this document does not define or
   require any specific client-side addressing schemes. 

   Like server-side resources on the Web, these presentation resources
   may support methods to query and change the state they hold. This
   model views the standard display of a MIME entity in a presentation
   resource as POSTing that entity to the resource. The implementation
   of the POST method typically creates a media handler for the entity
   (which does the actual work of display), pushes that media handler
   onto a navigation stack, and then gives the media handler access to
   the display area represented by the resource. 

   This model of the user-agent now allows us to describe an anchor
   more generally as a recipe for getting a MIME entity from a
   retrieval resource and posting it to a presentation resource. A
   link, as always, is a relation between two anchors (known as the
   tail and the head). Thus, links and anchors have the following
   properties: 

   link: 
       head anchor, tail anchor, relations 

   anchor: 
       source method call: 
           retrieval address, method, and argument(s) 

       target method call: 
           presentation address, method, and argument(s) 

   For this specification, the main point of the model is to prescribe
   what information must be made available to the media handlers by the

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   presentation resource (this affects the design of `plug-in' APIs).
   The key requirement we impose is that the media handler must have
   acccess to complete link information according to the link model
   just described. 

4. Geometry Negotiation

   The actual appearance of the compound presentation depends on a
   negotiation between the containing and contained presentations. This
   negotiation process may involve: 

   *   the respective style sheets of the entities presented 

   *   the nature of the media involved (resizable or not) 

   [This section is incomplete.] 

5. HTML Markup

   This specification defines an extension of the HTML 2.1 DTD [4].
   This description makes free use of SGML elements and entities
   defined there. 

5.1. Elements

   
   <!ELEMENT EMBED - - (PARAM*, CAPTION?, EMBED.BODY, CREDIT?)>
   <!ATTLIST EMBED
        %attrs;
        %link.internal;
        %link.metainfo;
        %case.metainfo;
        %size;                      -- size of reserved area --
        %align;                     -- alignment or float --
        %SDAPREF; '<Fig><Xref IDRef="#AttVal(SRC)"><?SDATrans  
Embed: #AttList>'
        %SDASUFF; '</Fig>'
        >
   

   Attributes: SRC, PARAMS; TITLE, URN, REL, REV; ACCEPT,
   ACCEPT-CHARSET, ACCEPT-ENCODING; WIDTH, HEIGHT; ALIGN, HSPACE,
   VSPACE, FLOWTO. 

   Common Attributes: ID, LANG, DIR (implied for all elements below). 

   From the point of view of the compound document architecture, the
   <EMBED> element has two purposes: 

   *   It conditionally creates a presentation resource, a child node
       in the presentation hierarchy below the presentation of the HTML
       entity which contained the <EMBED> element. 

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   *   It declares a link for which the target of the tail anchor
       implicitly uses the newly created presentation resource. 

   This functionality is implemented with the help of the <EMBED.BODY>
   and <PARAM> elements: 

   
   <!ELEMENT EMBED.BODY O O %A.content>
   <!ATTLIST EMBED.BODY
        %attrs;
        >
   

   The <EMBED> link is activated as soon as the parent presentation of
   the HTML entity is created. If for any reason the link fails, or the
   child presentation cannot be created, then the content of the
   <EMBED.BODY> element must be rendered in place of the <EMBED>
   element. 

      Note: There is never any need to actually include <EMBED.BODY>
      tags in a document; this construct exists solely to allow
      glitch-free use of %A.content in combination with <PARAM>
      tags. The %A.content was chosen in part to aid error recovery
      when the </EMBED> tag is accidentally omitted. 

   
   <!ELEMENT PARAM - O EMPTY  -- builds presentation specializer -- >
   <!ATTLIST PARAM
        %attrs;
        %key.value;
        %SDAPREF; "<?SDATrans Param: #AttList>"
        >
   

   Attributes: NAME, VALUE, ACCEPT, ACCEPT-CHARSET, ACCEPT-ENCODING. 

   The method invoked on the presentation resource is POST, which has
   two arguments. The first is the MIME [5] entity retrieved as a
   result of link activation. The second argument is a MIME entity
   called the presentation specializer, which is used to modify the
   resulting presentation, and is a generalization of the so-called
   "fragment id". This specializer can be defined in one of three ways,
   in order of priority: 

   <PARAM> elements 
       these elements collectively define a presentation specializer of
       type multipart/form-data, which is used by the media handler as

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       an unordered list of named parameters, each with full MIME type
       information 

   PARAMS attribute 
       defines a presentation specializer of type
       application/x-www-form-urlencoded, which is used by the media
       handler as an unordered list of named string parameters 

   "fragment id" of SRC attribute 
       defines a presentation specializer of type text/plain, which
       used as the name of the markup element on which the presentation
       should be focused 

   If two or more of the above mechanisms is used simultaneously, the
   one with higher priority wins. 

   
   <!ELEMENT INPUT - O EMPTY  -- builds retrieval specializer -- >
   <!ATTLIST INPUT
        %attrs;
        %key.value;                      -- key and MIME-typed value --
        TYPE       %InputType  #IMPLIED  -- defaults to TEXT in  
FORM context --
        CHECKED    (CHECKED)   #IMPLIED  -- initial boolean state --
        SRC        %URI;       #IMPLIED  -- embedded graphic for  
TYPE=IMAGE --
        %align.simple;                   -- alignment for TYPE=IMAGE --
        SIZE       CDATA       #IMPLIED
        MAXLENGTH  NUMBER      #IMPLIED
        %SDAPREF; "Input #AttVal(Type): "
        >
   

   Attributes: NAME, VALUE, ACCEPT, ACCEPT-CHARSET, ACCEPT-ENCODING;
   TYPE; CHECKED; SRC; ALIGN, SIZE, MAXLENGTH. 

   The <INPUT> element plays a largely parallel role to <PARAM>, but on
   the retrieval (source) end of the anchor rather than the
   presentation (target) end. Unfortunately, this element has been
   somewhat abused as a result of its great usefulness (though fixing
   these problems is beyond the scope of this proposal). 

   The only modification specified here is the addition of
   %mime.constraints; type information for the value of the input field
   (described in detail below). Note that the %mime.constraints;
   information is not intended to apply to the vestigial embedding link
   defined by the SRC attribute. 

   
   <!ELEMENT CAPTION - - (%text;)+ -- caption for floating element -->
   <!ATTLIST CAPTION
        %attrs;
        %align;                     -- side of rectangle where  
caption placed --

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        %SDAPREF; "Caption: "
        >
   

   Attributes: ALIGN, HSPACE, VSPACE, FLOWTO. 

   
   <!ELEMENT CREDIT - - (%text;)* -- copyright/credit for embedded  
object  -->
   <!ATTLIST CREDIT
        %attrs;
        %SDAFORM; "Fn"
        >
   

   In typical rendering with default attributes, the italicized,
   centered caption would be placed at the bottom of the area reserved
   for the child presentation, and the credit would appear as smaller
   roman text right-aligned near the bottom of the reserved area. 

   Because the caption is not an independent float, the ALIGN attribute
   must be interpreted somewhat differently. Here, the value of ALIGN
   indicates on which side of the reserved area the caption should be
   placed. The values MIDDLE and CENTER are not meaningful. 

   
   <![ %HTML.Highlighting [
   <!ENTITY % text  
"#PCDATA|A|EMBED|IMG|BR|%phrase|%font|SPAN|Q|BDO|SUP|SUB">
   ]]>
   <!ENTITY % text "#PCDATA|A|EMBED|IMG|BR|SPAN|Q|BDO|SUP|SUB">
   

   The <EMBED> tag is permitted as part of %text context, meaning that
   it does not force a paragraph break. The reason is that <EMBED>
   functions as either a float (alongside the text flow) or a glyph
   (part of the line of text line), but never as a block. 

5.2. Attribute Value Types

   HTML is an application of SGML for which application conventions
   play a strong role, particularly in the rich syntax of its attribute
   values. Although these conventions cannot be checked directly by the
   SGML parser, DTD "macros" can be used help flag these conventions
   for other validation tools. 

   
   <!ENTITY % Length    "CDATA"    -- number followed by optional  
units -->
   

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   Attributes values defining a length are uniformly specified as a
   number followed by an optional suffix denoting the units of
   measurement. The number must be an integer value or a floating-point
   real value such as 2.5 (scientific notation, as in 1.2e2, is not
   allowed). No white space is allowed between the number and the
   suffix. The default units are screen pixels, and the following
   suffixes may also be recognized: pt denotes points, pi denotes
   picas, in denotes inches, cm denotes centimeters, mm denotes
   millimeters, em denotes em units (in the default font), and px
   denotes screen pixels. 

   
   <!ENTITY % URI       "CDATA"    -- uniform resource identifier -->
   

   The syntax of Uniform Resource Identifiers is given by RFC 1630 [6].
   For historical reasons, attribute values of type %URI which identify
   retrieval resources may also include a final "fragment identifier",
   which is actually unrelated to the retrieval process and instead
   specifies a presentation specializer of type text/plain. 

5.3. Attribute Sets

   The HTML DTD does not make extensive use elements for structuring;
   instead it is typified by sets of related attributes which recur in
   multiple elements. In such a setting, DTD evolution can be
   facilitated by making consistent use of SGML "macros" to identify
   and reuse such attribute sets. 

   
   <!ENTITY % attrs
        'ID      ID         #IMPLIED   -- element identifier --
         LANG    NAME       #IMPLIED   -- RFC 1766 language tag --
         DIR     (ltr|rtl)  #IMPLIED   -- text directionality --'>
   

   The %attrs; attribute set is common to all elements. 

   ID 
       Used to define a document-wide identifier. This can be used for
       naming positions within documents as the destination of a
       hypertext link. An ID attribute value is an SGML NAME token.
       (NAME tokens are formed by an initial letter followed by
       letters, digits, "-" and "." characters. The letters are
       restricted to A-Z and a-z.) 

   LANG 
   DIR 
       These attributes are described in the HTML 2.1 specification

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       [4]. 

   
   <!ENTITY % link.internal  -- links with implicit presentation  
resource --
        'SRC     %URI;      #REQUIRED  -- resource to retrieve --
         PARAMS  CDATA      #IMPLIED   -- presentation specializer --'>
   

   SRC 
       The address of the resource to be retrieved in traversing this
       link. The address must be a valid URI [6]. 

   PARAMS 
       Declares a presentation method specializer argument of type
       application/x-www-form-urlencoded [1]. (It is strongly recommend
       to use `;' as a separator in place of `&'.) This attribute is
       suitable for declaring small, simple parameter lists; more
       general presentation specializers can be created using the
       <PARAM> mechanism. 

   
   <!ENTITY % mime.constraints  -- MIME typing constraints --
        'ACCEPT           CDATA  "text/plain" -- applicable media  
type(s) --
         ACCEPT-CHARSET   CDATA  "ISO-8859-1" -- appl. character  
encoding(s) --
         ACCEPT-ENCODING  CDATA  "ISO-8859-1" -- appl. transfer  
encoding(s) --'>
   

   ACCEPT 
   ACCEPT-CHARSET 
   ACCEPT-ENCODING 
       In link-related elements, the %mime.constraints attribute-set
       provides an advisory range of MIME type information, suggesting
       to the data producer what is acceptable to the consumer, and to
       the consumer what is available from the producer. This
       attribute-set also provides specific defaults for situations in
       which the MIME type information of the corresponding data cannot
       otherwise be determined. 

       The attribute values have syntax identical to the corresponding
       HTTP headers in HTTP/1.1 [7], except that the first entry of
       each list must be a definite (non-wildcarded) type
       specification. This first entry is used as default type
       information. 

       As attributes of linking elements (such as <EMBED>), these
       constraints support efficient type-negotiation, by narrowing the
       range of types in advance. For instance, a link with
       ACCEPT="video/mpeg, video/*" suggests that multiple video
       formats may be available for negotiation; if the the only video
       format that the user-agent can usefully interpret is video/avi,
       a reasonable HTTP negotiation offer would be Accept: video/avi,
       video/mpeg (with a video/mpeg result stored to disk). 


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       In link specializer elements defining name-value pairs (<INPUT>
       and <PARAM>), these attributes specify the parameter types that
       should be acceptable to the method and resource in question
       (retrieval and presentation resource, resp.). When the user is
       allowed to change the types of the values (as is permitted by
       RFC 1867 [8] for <INPUT>s of TYPE=FILE in <FORM> links, for
       example), these attributes may therefore be used constrain the
       user's changes, as a service to the eventual data consumer. In
       addition, the ACCEPT attribute (but not ACCEPT-CHARSET or
       ACCEPT-ENCODING) should be used to interpret the default value
       provided in the markup (the VALUE attribute). 

       
   Note on MIME/SGML/I18N/FORM Interactions: Resources which rely on
   form submission (or presentation specializers) in an
   internationalized setting must be aware of one important caveat:
   just because the default value (the VALUE attribute) of a field is
   returned, it cannot be guaranteed to be returned as an identical
   sequence of octets -- only as (another) valid encoding of the same
   sequence of characters (under the character- and transfer-encodings
   declared for the field in the form submission). 

   This may be true even if the character- and transfer-encodings of
   the original HTML MIME entity containing the form are the same as
   the eventual encodings used for form submission. The reason is that
   the MIME-to-SGML process is not required to preserve any of the
   encoding information of the MIME entity, only the actual sequence of
   characters in the (abstract) document character set of the SGML
   document entity. [This note should be moved to the HTML I18N draft.] 

   
   <![ %HTML.Recommended [
        <!ENTITY % key.value  -- key/value pair with typed value --
                'NAME    NAME       #IMPLIED   -- keyword, usually  
required --
                 VALUE   CDATA      #IMPLIED   -- default value --
                 %mime.constraints;            -- type constraints  
for value --'>
   ]]>
   <!ENTITY % key.value  -- key/value pair with typed value --
        'NAME    CDATA      #IMPLIED   -- keyword, usually required --
         VALUE   CDATA      #IMPLIED   -- default value --
         %mime.constraints;            -- type constraints for  
value --'>
   

   NAME 
       A parameter name. It is not required that NAMEs be unique keys,
       even within the scope of a single linking element; a
       multiple-values model is supported. With the strict HTML DTD,
       this attribute is a case-insensitive SGML NAME token. With the

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       default HTML DTD, this attribute may be arbitrary CDATA, and any
       desired case-folding is the responsibility of the eventual
       consumer. 

   VALUE 
       Tags with %key.value attributes are used to collectively build
       up link specializers. Abstractly, the WWW method specializer
       resulting from such a collection of tags is the MIME entity of
       type multipart/form-data [8] which encodes all the name-value
       pairs and their types. Any %key.value tags without NAME
       attributes do not contribute to the specializer. 

       There are two exceptions to this general model of specializer
       construction. The first is historical: in the case of <INPUT>
       tags in a <FORM>, the default is to create a specializer of type
       application/x-www-form-urlencoded (this can only unambiguously
       create a form submission with values having default MIME type).
       This historical behavior can be overridden by setting the
       <FORM>'s ENCTYPE attribute to "multipart/form-data" (which is
       strongly recommended). In case of legacy servers which require
       application/x-www-form-urlencoded form submission, but expect
       values of non-default MIME types, those expectations must be
       explicitly spelled out in the %mime.constraints. of each
       <INPUT>. 

       The second exception provides a way to build small method
       specializers of arbitrary character-based type; the rule is that
       if exactly one instance of a %key.value tag is supplied, and
       that tag does not have a NAME attribute, then its VALUE is the
       entire specializer entity. 

   
   <!ENTITY % link.metainfo  -- overall link metainfo --
        'TITLE   CDATA      #IMPLIED   -- overall title for resource --
         URN     %URI       #IMPLIED   -- global entity name --
         REL     %linkType  #IMPLIED   -- link relationship --
         REV     %linkType  #IMPLIED   -- reverse relationship --'>
   <![ %HTML.Deprecated [
        <!ENTITY % case.metainfo  -- link metainfo for specific  
variant --
                '%mime.constraints;             -- type constraints --
                 METHODS  NAMES     #IMPLIED    -- deprecated --'>
   ]]>
   <!ENTITY % case.metainfo  -- link metainfo for specific variant --
        '%mime.constraints;             -- type constraints --'>
   <!ENTITY % linkExtraAttributes
        '%link.metainfo;
         %case.metainfo;
        '>
   

   The link meta-information attribute set %linkExtraAttributes has

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   been logically split into two parts to ease future extension. The
   new %case.metainfo component represents meta-information specific to
   a single link variant (currently consisting of MIME type
   information). 

   The only other change is the deprecation of the METHODS attribute.
   It is difficult to define the semantics of the METHODS attribute in
   an object-oriented model of the Web. 

   
   <!ENTITY % size
        'WIDTH   %Length    #IMPLIED   -- desired width in units --
         HEIGHT  %Length    #IMPLIED   -- desired height in units --'>
   

   The %size; attribute set is used for elements that require
   bounding-box hints for rapid layout. 

   
   <!ENTITY % align.simple
        'ALIGN   (top|middle|bottom) #IMPLIED  -- glyph alignment --'>
   <!ENTITY % align
        'ALIGN   (top|middle|bottom|left|center|right)  #IMPLIED
         HSPACE  %Length    #IMPLIED   -- text stays this far away  
horizontally --
         VSPACE  %Length    #IMPLIED   -- text stays this far away  
vertically --
         FLOWTO  NAME       #IMPLIED   -- flow text around block  
until this ID --'>
   

   The %align; attribute set is used for graphic elements that can
   function either as floats or glyphs, while %align.simple; is used
   for graphic elements that function only as glyphs. 

   ALIGN 
       The value of this attribute indicates the relation between the
       graphic element and the text flow. The possible values are: 

       
       LEFT 
       RIGHT 
           The graphic element functions as a float; subsequent text
           will be flowed around the graphic. The left (resp. right)
           edge of the graphic is aligned with the left (resp. right)
           edge of the text flow area. 

       CENTER 
           The graphic element functions as a float centered
           horizontally within the text flow area. The graphic
           interrupts the flow of text, but does not logically break

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           the current paragraph. 

       TOP 
       BOTTOM 
           The graphic element functions as a glyph within the line of
           text. The top (resp. bottom) of the graphic is aligned with
           the highest point (resp. baseline) of the rendered line of
           text. 

       MIDDLE 
           The graphic element functions as a glyph within the line of
           text. The baseline of the graphic is aligned with the
           baseline of the rendered line of text. By default, the
           baseline of the image is taken to be at its vertical center.
           (There is currently no way to override this default
           baseline.) 

   HSPACE 
   VSPACE 
       The text surrounding the graphic element remains at least this
       far away. HSPACE specifies the minimum horizontal distance
       between the text and the left/right edges of the graphic, while
       VSPACE specifies the minimum vertical distance between the text
       and its top/bottom edges. This padding does not offset the
       alignment in any way. 

   FLOWTO 
       Sometimes it is important that some subsequent element appear
       below a float, rather than as part of the text flow alongside
       it; the ID of that element can be specified with the FLOWTO
       attribute. 

       This attribute provides a more structured alternative to <BR
       CLEAR=ALL>. Its purpose is to declare the range of text with
       which the float is associated (from the element itself, until
       the element whose ID is given by FLOWTO). In typical rendering,
       the text beyond the specified range would not be flowed around
       the graphic. 

6. Summary of DTD Changes for HTML

   The resulting SGML doctype is provisionally called "-//IETF//DTD
   HTML 2.1E//EN", and is defined in terms of the doctype "-//IETF//DTD
   HTML 2.1//EN". Because the HTML 2.1 DTD [4] defines multibyte entity
   references, it must be used with an SGML declaration that provides
   an extended coded character set; the HTML 2.1E DTD inherits this
   restriction. 

6.1. Retroactive Modifications for HTML 2.1

   The following changes are included below, but we recommend that they
   be included directly into future drafts of HTML 2.1, for
   consistency. This section will be removed from future versions of

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   this draft, once these issues are resolved. 

   CHARSET in %linkExtraAttributes 
       Change to ACCEPT-CHARSET to allow content negotiation. 

   ACCEPT-CHARSET in <FORM> 
       Echoing comments of Larry Masinter, drop for now. This is wrong
       because it conflicts with media type metainfo for the FORM link,
       which should have priority since client Accept-ance was invented
       first! An ACCEPT-CHARSET should be given to each relevant INPUT
       instead. 

   ACCEPT in <INPUT> 
       Add ACCEPT-CHARSET and ACCEPT-ENCODING here for constraining
       input values (not in <FORM> element). 

   METHODS in %linkExtraAttributes 
       This should eventually be replaced by a per-link-variant METHOD
       attribute; multiple methods may have conflicting type
       signatures, not to mention semantics and security implications. 

6.2. Incremental DTD for HTML 2.1E

   The proposed HTML 2.1E DTD may be constructed unambiguously from the
   HTML 2.1 DTD [4] and the DTD fragment below by the following
   procedure: Entity and element definitions already in HTML 2.1 are
   overridden (respecting the marked sections); all others are added to
   the end of the DTD in the order they appear here. 

   
   <!-- This DTD fragment shows only changes from HTML 2.1 to 2.1E -->
   
   <!ENTITY % HTML.Version "-//IETF//DTD HTML 2.1E//EN">
   
   <!-- Content models -->
   
   <![ %HTML.Highlighting [
   <!ENTITY % text  
"#PCDATA|A|EMBED|IMG|BR|%phrase|%font|SPAN|Q|BDO|SUP|SUB">
   ]]>
   <!ENTITY % text "#PCDATA|A|EMBED|IMG|BR|SPAN|Q|BDO|SUP|SUB">
   
   <!-- Attribute value types -->
   
   <!ENTITY % URI       "CDATA"    -- uniform resource identifier -->
   <!ENTITY % Length    "CDATA"    -- number followed by optional  
units -->
   
   <!-- Attribute sets -->
   
   <!ENTITY % attrs
        'ID      ID         #IMPLIED   -- element identifier --
         LANG    NAME       #IMPLIED   -- RFC 1766 language tag --

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         DIR     (ltr|rtl)  #IMPLIED   -- text directionality --'>
   <!ENTITY % size
        'WIDTH   %Length    #IMPLIED   -- desired width in units --
         HEIGHT  %Length    #IMPLIED   -- desired height in units --'>
   <!ENTITY % align.simple
        'ALIGN   (top|middle|bottom) #IMPLIED  -- glyph alignment --'>
   <!ENTITY % align
        'ALIGN   (top|middle|bottom|left|center|right)  #IMPLIED
         HSPACE  %Length    #IMPLIED   -- text stays this far away  
horizontally --
         VSPACE  %Length    #IMPLIED   -- text stays this far away  
vertically --
         FLOWTO  NAME       #IMPLIED   -- flow text around block  
until this ID --'>
   <!ENTITY % mime.constraints -- MIME typing constraints --
        'ACCEPT           CDATA  "text/plain" -- applicable media  
type(s) --
         ACCEPT-CHARSET   CDATA  "ISO-8859-1" -- appl. character  
encoding(s) --
         ACCEPT-ENCODING  CDATA  "ISO-8859-1" -- appl. transfer  
encoding(s) --'>
   <!ENTITY % link.internal -- links with implicit presentation  
resource --
        'SRC     %URI;      #REQUIRED  -- resource to retrieve --
         PARAMS  CDATA      #IMPLIED   -- presentation specializer --
         USEMAP  %URI       #IMPLIED   -- default event handling,  
if needed --'>
   <!ENTITY % link.metainfo  -- overall link metainfo  --
        'TITLE   CDATA      #IMPLIED   -- overall title for resource --
         URN     %URI       #IMPLIED   -- global entity name --
         REL     %linkType  #IMPLIED   -- link relationship --
         REV     %linkType  #IMPLIED   -- reverse relationship --'>
   <![ %HTML.Deprecated [
        <!ENTITY % case.metainfo  -- link metainfo for specific  
variant --
                '%mime.constraints;             -- type constraints --
                 METHODS  NAMES     #IMPLIED    -- deprecated --'>
   ]]>
   <!ENTITY % case.metainfo  -- link metainfo for specific variant --
        '%mime.constraints;             -- type constraints --'>
   <!ENTITY % linkExtraAttributes
        '%link.metainfo;
         %case.metainfo;
        '>
   <![ %HTML.Recommended [
        <!ENTITY % key.value  -- key/value pair with typed value --
                'NAME    NAME       #IMPLIED   -- keyword, usually  
required --
                 VALUE   CDATA      #IMPLIED   -- default value --
                 %mime.constraints;            -- type constraints  
for value --'>
   ]]>
   <!ENTITY % key.value  -- key/value pair with typed value --
        'NAME    CDATA      #IMPLIED   -- keyword, usually required --
         VALUE   CDATA      #IMPLIED   -- default value --
         %mime.constraints;            -- type constraints for  
value --'>
   
   <!-- Embed and Related Elements -->
   
   <!ELEMENT EMBED - - (PARAM*, CAPTION?, EMBED.BODY, CREDIT?)>
   <!ATTLIST EMBED
        %attrs;
        %link.internal;
        %link.metainfo;

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        %case.metainfo;
        %size;                      -- size of reserved area --
        %align;                     -- alignment or float --
        %SDAPREF; '<Fig><Xref IDRef="#AttVal(SRC)"><?SDATrans  
Embed: #AttList>'
        %SDASUFF; '</Fig>'
        >
   <!ELEMENT CAPTION - - (%text;)+ -- caption for floating element -->
   <!ATTLIST CAPTION
        %attrs;
        %align;                     -- side of rectangle where  
caption placed --
        %SDAPREF; "Caption: "
        >
   <!ELEMENT EMBED.BODY O O %A.content>
   <!ATTLIST EMBED.BODY
        %attrs;
        >
   <!ELEMENT CREDIT - - (%text;)* -- copyright/credit for embedded  
object  -->
   <!ATTLIST CREDIT
        %attrs;
        %SDAFORM; "Fn"
        >
   
   <!-- Link Specializer Elements -->
   
   <!ELEMENT INPUT - O EMPTY  -- builds retrieval specializer -- >
   <!ATTLIST INPUT
        %attrs;
        %key.value;                      -- key and MIME-typed value --
        TYPE       %InputType  #IMPLIED  -- defaults to TEXT in  
FORM context --
        CHECKED    (CHECKED)   #IMPLIED  -- initial boolean state --
        SRC        %URI;       #IMPLIED  -- embedded graphic for  
TYPE=IMAGE --
        %align.simple;                   -- alignment for TYPE=IMAGE --
        SIZE       CDATA       #IMPLIED
        MAXLENGTH  NUMBER      #IMPLIED
        %SDAPREF; "Input #AttVal(Type): "
        >
   <!ELEMENT PARAM - O EMPTY  -- builds presentation specializer -- >
   <!ATTLIST PARAM
        %attrs;
        %key.value;
        %SDAPREF; "<?SDATrans Param: #AttList>"
        >
   
   <!-- Corrections -->
   
   <!ELEMENT FORM - - %body.content -(FORM) +(INPUT|SELECT|TEXTAREA)>
   <!ATTLIST FORM
        %attrs;
        ACTION CDATA #IMPLIED
        METHOD (%HTTP-Method) GET
        ENCTYPE %Content-Type; "application/x-www-form-urlencoded"
        %SDAPREF; "<Para>Form:</Para>"
        %SDASUFF; "<Para>Form End.</Para>"

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        >
   

7. Transition Issues

7.1. Sun's APPLET Element

   Sun's <APPLET> element [9] can be mapped to <EMBED> with essentially
   just a few name changes: 

   *   APPLET becomes EMBED 

   *   NAME becomes ID 

   *   CODE is prefixed with CODEBASE to become SRC 

   *   CODEBASE is not needed (the embedding API is required to pass
       the retrieval URI (the SRC) to the media handler, from which the
       CODEBASE can be deduced) 

   It is possible to construct peculiar examples where the CODEBASE is
   not the base URI of the combination of CODEBASE with CODE, but this
   seems more confusing than useful. 

7.2. Netscape's Early Implementations of EMBED

   Netscape initially implemented <EMBED> as an empty element [10], not
   a container. While this causes some forward-compatibility problems,
   the %A.content content model of <EMBED.BODY> allows error recovery
   to occur after the first paragraph break when </EMBED> is omitted. 

   Netscape's initial implementation also uses arbitrary attributes to
   pass parameters. In the interim, authors can use a combination like
   the following for compatibility (though it's still not legal SGML): 

   
        <EMBED SRC="sample.app" ALT="simple alt text" FOO=3 BAR=9>
                <PARAM NAME=foo VALUE=3> <PARAM NAME=bar VALUE=9>
        </EMBED>
   

8. Security Considerations

   Immediate invocation of link without user feedback or control
   aggravates security problems of links described in [11]. 

9. Acknowledgments

   Dave Raggett did a lot of the early work on <FIG> [12] which is
   reflected here in <EMBED>. 

   Arthur van Hoff of Sun Microsystems designed <APPLET> based on
   discussion in the hotjava-interest mailing list. This proposal has

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   been strongly influenced by the <APPLET> tag. 

   Dan Connolly began work to formalize the Web model [13]. Discussions
   with Larry Masinter and Gavin Nicol were also important in shaping
   the model presented here. 

10. References

   1.  T. Berners-Lee and D. Connolly, HTML 2.0, RFC 1866. 

   2.  J. Postel, Internet Media Types, RFC 1590. 

   3.  R. Fielding, Relative Uniform Resource Locators, RFC 1808. 

   4.  F. Yergeau, G. Nicol, G. Adams, and M. Duerst, HTML
       Internationalization. 

   5.  N. Borenstein and N. Freed, MIME Content Types, RFC 1521. 

   6.  T. Berners-Lee, Universal Resource Identifiers, RFC 1630. 

   7.  T. Berners-Lee, R. Fielding, and H. F. Nielsen, HTTP/1.1. 

   8.  Larry Masinter et al., Form-Based File Upload in HTML, RFC 1867. 

   9.  Sun Microsystems, APPLET Element Syntax. 

   10. Netscape Communications, Netscape Navigator 2.0 Feature
       Descriptions 

   11. T. Berners-Lee, L. Masinter, and M. McCahill, Uniform Resource
       Locators, RFC 1738. 

   12. Dave Raggett, HTML3 Draft. 

   13. Daniel W. Connolly, Resource Discovery and Reliable Links. 

11. Authors' Addresses

   
      Paul Burchard
      <burchard@cs.princeton.edu>
      Computer Science Dept.
      Princeton University
      35 Olden Street
      Princeton NJ 08544 

   
      Dave Raggett
      <dsr@w3.org>
      World Wide Web Consortium

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      Massachusetts Institute of Technology
      545 Technology Square
      Cambridge MA 02139 


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