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



HTML and Style Sheets                                              22 Nov 1995

   INTERNET DRAFT                                 Bos, Raggett & Lie,
   Expires in six months                          World Wide Web Consortium

                            HTML and Style Sheets

                        <draft-ietf-html-style-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
   directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
   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.ietf.org/ 

   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. 

   This specification is also available via the Web in hypertext form
   as a Working Draft of the World Wide Web Consortium, see:
   http://www.w3.org/pub/WWW/TR 

   Authors:
       Bert Bos <bert@w3.org>,
       Dave Raggett <dsr@w3.org>,
       Hekon Lie <howcome@w3.org> 

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Abstract

   The HyperText Markup Language (HTML) is a simple markup language
   used to create hypertext documents that are portable from one
   platform to another. HTML documents are SGML documents with generic
   semantics that are appropriate for representing information from a
   wide range of applications. This specification extends HTML to
   provide support for rendering instructions expressed in separately
   specified notations. It is no longer necessary to extend HTML when
   new forms of rendering instructions are needed. Rendering
   instructions can be included with individual HTML elements to which
   they apply, or grouped together in the document head, or placed in

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   associated style sheets. This specification does not specify
   particular style sheet notations, leaving that to other
   specifications. 

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Contents

   *   Associating documents with style sheets  ........................... 2

   *   Media Dependencies  ................................................ 3

   *   The LINK Tag  ...................................................... 3

   *   The STYLE Tag  ..................................................... 4

   *   Common Attributes  ................................................. 7

   *   The C tag  ......................................................... 8

   *   User interface and user supplied style sheets 

   *   Deployment Issues  ................................................. 9

   *   Performance Issues  ............................................... 10

   *   References  ....................................................... 11

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Associating HTML documents with Style Sheets

   There are several approaches for associating HTML documents with
   separate style sheets: 

   User applied style sheets 
       The user agent may provide the means for users to select and
       apply style sheets.

       
   Implicit associations 
       The appropriate style sheet may be implied by the URL or other
       information describing the resource. This approach allows style
       sheets to be retrieved in advance of, or at the same time as,
       the HTML document itself. Implicit associations are not defined
       in this report.

       
   Explicit associations 
       The author can specify one or more alternative style sheets for
       an HTML document using one of the methods described below.

       
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   In HTML it is also possible to put style sheets in-line in the
   document. HTML is extended with a new element and a new attribute
   (both called STYLE), as described below. No matter how style sheets
   are applied, the user should be made aware that a particular style
   is in force and should have the option of turning it off. 

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Media Dependencies

   Styles may often be designed for a restricted range of media, e.g.
   for graphical user interfaces with scalable fonts and millions of
   colors; for A4 paper media; for speech output; or for simple
   terminals with fixed pitch single font and 80x24 character displays.
   This proposal doesn't provide an explicit means to state the
   conditions under which a given style sheet is applicable. 

   Style sheet notations may themselves provide support for media
   dependencies. Another approach is to use a generic URL to reference
   a style sheet, and to make the binding to a specific URL according
   to the media required. This will be described in a separate working
   draft. 

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Using the HTML LINK element

   In HTML, the LINK element is used to create a typed hyperlink
   between the document and some other resource. The REL attribute
   defines the type of the link. With REL=stylesheet, the LINK element
   can also be used to link to a style sheet. 

   Authors can use LINK elements to offer readers a choice of style
   sheets, e.g: 

       <LINK TITLE="Traditional" REL=stylesheet HREF="old.style">
       <LINK TITLE="Modern" REL=stylesheet HREF="modern.style">
       <LINK TITLE="Wacky" REL=stylesheet HREF="wacky.style">
       <TITLE>ACME Widgets Corp</TITLE>
   
       <H1>ACME Widgets Corp</H1>
       <P>If your browser supports style sheets, try our new look
       in traditional, modern and wacky styles.
       
       ...

   Another approach is to use a generic URL for a LINK which maps to a
   set of alternative style sheets. A separate working draft will
   describle how resource descriptions can be used for this purpose. 

   This specification builds upon the definition of the LINK element in
   HTML 2.0 in the following respects: 

       <!ELEMENT LINK - O EMPTY>
       <!ATTLIST LINK

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           href    CDATA     #REQUIRED  -- Universal Resource Locator --
           title   CDATA     #IMPLIED   -- advisory title string --
           rel     CDATA     #IMPLIED   -- forward link type --
           rev     CDATA     #IMPLIED   -- reverse link type --
           media   CDATA     #IMPLIED   -- Internet media type --
           >

   *   The forward link type "stylesheet" is hereby defined to signify
       that the associated LINK element specifies a link to a style
       sheet that may be applied to the HTML document containing the
       LINK element. The HREF attribute specifies the network address
       of the linked style sheet.

       
   *   If there are several such links, then these are considered as
       providing a choice of alternative style sheets. The character
       string supplied with the TITLE attribute is recommended for use
       in building a menu of alternative styles. 

       Note that the order of such LINK elements in the document markup
       does not signify preference order!

       
   *   The MEDIA attribute may be used to specify the Internet Media
       type and associated parameters for the linked style sheet. This
       allows the user agent to disregard style sheets in unsupported
       notations, without the need to first make a remote query across
       the network. 

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The STYLE element

   A single STYLE element may be included in the document head. It
   allows authors to include style rules within the HTML document, e.g. 

       <HEAD>
       <TITLE>Title</TITLE>
       <STYLE NOTATION="application/css" SRC="weird.css">
         H1 { color: brown }
         P  { color: blue  }
       </STYLE>
       </HEAD>

   In the example, the CSS notation is used. The STYLE element
   specifies color overrides to the "weird.css" style sheet for H1 and
   P elements. The STYLE element is formally defined by: 

       <!ELEMENT style - O (#PCDATA)>
       <!ATTLIST style
           notation CDATA    #REQUIRED -- Internet media type for style --
           src      CDATA    #IMPLIED  -- URL for separate style sheet --

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           title    CDATA    #IMPLIED  -- advisory title for this style --
           >

   The attributes are defined as follows: 

   NOTATION 
       This required attribute defines the style notation as an
       Internet Media type including associated parameters. It is used
       in exactly the same way as with LINK elements. The notation
       applies to an external style sheet linked via the SRC attribute
       and to rendering annotations on elements in the document body
       attached with the STYLE attribute.

       
   SRC 
       This may be used to specify the URL for a base style sheet. The
       style sheet must use the same notation as rendering instructions
       in the STYLE element, and will normally be overridden by those
       instructions or subsequent STYLE attributes in the document
       body. 

       This attribute is needed for the extremely common case where a
       linked style sheet is overridden by a few rules in the style
       element or by properties given by the style attribute on
       particular elements. In the absence of the SRC attribute you
       would need to include the style element, a link element and a
       means for selecting which link element to use.

       
   TITLE 
       The user agent is recommended to use the title string when
       building a menu of alternative style sheets. This will only
       happen if the STYLE element occurs together with linked style
       sheets as specified by one or more LINK elements. In the absence
       of such LINK elements, the TITLE attribute may be used to
       describe the style sheet for the purpose of allowing the user to
       turn style sheets on and off.

       
   CSS supports the ability to cascade several style sheets so that
   their effects are blended together. The STYLE element can be used
   with CSS to cascade style sheets using the CSS @import command, e.g. 

       <HEAD>
       <TITLE>Title</TITLE>
       <STYLE NOTATION="application/css">
         @import "house-style.css"
         @import "draft-report.css"
         H1 { color: red }     -- override cascaded style sheets --
       </STYLE>

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       </HEAD>

   When the STYLE element occurs together with one or more LINK
   elements that specify linked style sheets, the user agent should
   consider the STYLE element in preference to the LINK elements. 

   The content model for the STYLE element precludes SGML tags, and the
   end tag of a STYLE element can usually be omitted, e.g. when the
   STYLE element is followed by another element. Instances of the
   characters "&", "<" or ">" within rendering instructions should be
   escaped using SGML entities, e.g. &amp; &lt; and &gt; respectively. 

   Note that if we later decide to allow multiple STYLE elements in the
   document head, e.g. to cater for alternative styles, then we will
   need a different way of specifying the style notation in use for
   STYLE attributes for elements in the document body. The suggested
   choice is an attribute on the BODY element, e.g. "stylenotation",
   with the same definition as the "notation" attribute for the STYLE
   element. 

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Common Attributes

   To support effective use of style sheets with HTML documents a
   number of common attributes are proposed. These can be used with
   most HTML elements. In general, all attribute names and values in
   this specification are case insensitive, except where noted
   otherwise. 

   
   <!ENTITY % attrs
          "id      ID       #IMPLIED  -- element identifier --
           class   NAMES    #IMPLIED  -- for subclassing elements --
           style   CDATA    #IMPLIED  -- rendering annotation --
           lang    NAME     #IMPLIED  -- as per RFC 1766 --
           dir   (ltr|rtl)  #IMPLIED  -- I18N text direction --">
   

   ID 
       Used to define a document-wide identifier. This can be used for
       naming positions within documents as the destination of a
       hypertext link. It may also be used by style sheets for
       rendering an element in a unique style. 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. 

   CLASS 
       A space separated list of SGML NAME tokens. CLASS names specify
       that the element belongs to the corresponding named classes.
       These may be used by style sheets to provide class dependent
       renderings. 

   STYLE 
       A text string providing rendering information specific to this
       element. The notation is specified with the STYLE element in the
       document head. The default notation is hereby defined to be
       "application/css". 

       For example: 

           <TITLE>Test Document</TITLE>
           <STYLE NOTATION="application/css">
           <P STYLE="color: red; font-style: small-caps">This text should
            be in small capitals and colored red!

       The end tag for the STYLE element has been omitted here since
       the element is unambigously ended by the <P> start tag.

       
   LANG 
       A LANG attribute identifies the natural language used by the
       content of the associated element.The syntax and registry of

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       language values are defined by RFC 1766. In summary the language
       is given as a primary tag followed by zero or more subtags,
       separated by "-". White space is not allowed and all tags are
       case insensitive. The name space of tags is administered by
       IANA. The two letter primary tag is an ISO 639 language
       abbreviation, while the initial subtag is a two letter ISO 3166
       country code. Example values for LANG include: 

             en, en-US, en-uk, i-cherokee, x-pig-latin.

   DIR 
       Human writing systems are grouped into scripts, which determine
       amongst other things, the direction the characters are written.
       Elements of the Latin script are nominally left to right, while
       those of the Arabic script are nominally right to left. These
       characters have what is called strong directionality. Other
       characters can be directionally neutral (spaces) or weak
       (punctuation). 

       The DIR attribute specifies an encapsulation boundary which
       governs the interpretation of neutral and weakly directional
       characters. It does not override the directionality of strongly
       directional characters. The DIR attribute value is one of LTR
       for left to right, or RTL for right to left, e.g. DIR=RTL. 

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The C Tag

       <!ELEMENT c O O (%text)*>
       <ATTLIST c
               %attrs;     -- id, class, style, lang and dir --
               >

   Sometimes it is desirable to apply a style to some text which
   doesn't have a structural role. For instance, the first few letters
   or words after a drop down capital may be rendered as small capital
   letters. In such situations it is inappropriate to use an existing
   tag such as <EM>. On existing user agents, the first letter would
   appear normally, but the next few would be mysteriously italicized.
   The new <C> tag is recommended instead, as it has no effect on
   existing user agents. 

   An example based on CSS: 

       <TITLE>Title</TITLE>
       <STYLE NOTATION="application/css">
       P { 
          text-effect: drop-cap,
          font-size: 12pt,
          alt-font-size: 24pt      -- assuming leading is zero --
         }
       C { font-style: small-caps }
       </STYLE>

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       <P>T<C>he first</C> few words of an article in The Economist..

   This would be formatted to look something like: 

       ___
        | HE FIRST few words
        | of an article in the
       Economist..

   While on an existing user agent it would look like: 

       The first few words of an
       article in the Economist..

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User interface and user supplied style sheets

   In an interactive user agent, if an external (i.e., not built into
   the user agent or supplied by the user) style is being applied, the
   user should be made aware of it and be given the option of turning
   it off, or of selecting a different style, either for this document
   only or for all future documents as well. A flag in the corner of
   the window or an option button in the menubar should be enough. 

   It may be possible for the user to combine several of the available
   style sheets. At least it should be possible for the user to choose
   a personal style instead of, or maybe in combination with, external
   style sheets. 

   When a user agent applies a style sheet to a document while the
   author of that document has indicated a preference for a different
   style sheet, the user agent may have to alert the user to that fact.
   Exactly how and when that is done is outside the scope of this
   report. E.g., the CSS style sheet language gives precise rules for
   the conditions under which a user is allowed to override the
   author's choices. 

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Deployment Issues

   The introduction of style sheets will give authors and users much
   greater control over the appearence of documents. It will also
   remove the pressure on vendors to extend HTML itself when ever a new
   rendering feature is needed. However, it will take time for
   widespread deployment of support for style sheets. What are some of
   the deployment issues? 

   Traditionally, HTML user agents have silently ignored unknown start
   and end tags, or unknown attributes. Very few user agents support
   the LINK element. As as result: 

   *   The use of LINK for style sheets will be ignored 


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   *   The STYLE attribute will be ignored 

   *   The STYLE element will be ignored, but its contents will be
       treated as part of the document body, and rendered as such. 

   As a result it is recommended that during the transition phase,
   authors avoid placing rendering instructions within the STYLE
   element. This leaves authors free to place rendering instructions
   within STYLE attributes on specific HTML elements and to use
   separate style sheets, linked via the STYLE element or one or more
   LINK elements. 

   This restriction may be lifted if vendors adopt another proposal by
   the World Wide Web Consortium for specifying variants of network
   resources. The resource variants mechanism would allow authors to
   serve up two versions of a document, one designed for old user
   agents and one for new user agents. The choice is made by the user
   agent based on descriptions of resource variants. The resource
   variants mechanism is the subject of a related working draft. 

   
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Performance Issues

   Some people have voiced concerns over performance issues for style
   sheets. For instance, if the user agent has to wait until it has
   finished down loading lengthy style sheets, before it can start to
   display a document, then users will start to complain. A similar
   situation arises if the document head includes a lengthy set of
   style rules. 

   The current proposal sidesteps these issues, by allowing authors to
   include rendering instructions within each HTML element. The
   rendering information is then always available by the time the user
   agent wants to render each element. 

   In many cases, authors will take advantage of a common style sheet
   for a group of documents. In this case, distributing rendering
   information through out the document will actually lead to worse
   performance than using a linked style sheet, since for most
   documents, the style sheet will already be present in the local
   cache. The public availability of good style sheets will encourage
   this effect. 

   The ability to override style sheets with information in the
   document head, or on individual HTML elements, increases the
   effectiveness of the local cache. Small changes to the document
   style can be kept out of the common style sheet, thereby allowing
   the same style sheet to be used with more documents, which in turn
   increases the chances of finding it in the cache. 

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References

   RFC 1866 
       "Hypertext Markup Language - 2.0" by T. Berners-Lee & D.
       Connolly, November 1995. This document can be downloaded from
       ftp://ds.internic.net/rfc/rfc1866.txt. 

   RFC 1766 
       "Tags for the Identification of Languages", by H. Alvestrand,
       UNINETT, March 1995. This document can be downloaded from
       ftp://ds.internic.net/rfc/rfc1766.txt. 

   CSS (5th draft) 
       "Cascading style sheets" by Hekon Lie & Bert Bos, November 1995.
       This document can be downloaded from
       http://www.w3.org/hypertext/WWW/Style/css/draft5.html 

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    The World Wide Web Consortium: http://www.w3.org/ 


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