WD-style-970324WD-style-970324This draft is work under review by the W3C HTML Working Group, for potential incorporation in an upcoming version of the HTML specification, code named Cougar. Please remember this is subject to change at any time, and may be updated, replaced or obsoleted by other documents. It is inappropriate to use W3C Working Drafts as reference material or to cite them as other than "work in progress".
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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 style rules expressed in separately specified notations. It is no longer necessary to extend HTML when new presentation styles are needed. Style rules can be (a) included with individual HTML elements to which they apply, (b) grouped together in the document head, or (c) placed in associated style sheets. This specification does not specify particular style sheet notations, leaving that to other specifications.
To make it easier to apply a style to parts of a document, two new elements for use in the body of an HTML document are defined: DIV and SPAN. The first is to enclose a division (chapter, section, etc.) of a document, making it possible to give a whole section a distinctive style. The latter is used within paragraphs, similarly to EM, but in cases where none of the other HTML elements (EM, STRONG, VAR, CODE, etc.) apply.
This specification proposes extensions to HTML with support for the following features:
Style sheets, by contrast, apply to specific media or media groups. A style sheet intended for screen use, may be applicable when printing, but is of little use for speech-based browsers. This specification allows you to define the broad categories of media a given style sheet is applicable to. This allows user agents to avoid retrieving inappropriate style sheets. Style sheet languages may include features for describing media dependencies within the same style sheet.
The following introduces the mechanisms used by HTML to associate HTML documents with style sheets. Further details are given later on with the formal description of the elements.
The LINK element allows you to reference a style sheet in a separate file, e.g.
<HEAD>
<LINK REL=stylesheet HREF="housestyle.css">
...
The HREF attribute gives a URL for the style sheet. The REL attribute is used to indicate that the LINK is to a style sheet. The STYLE element is used when you want to include a style sheet as part of the document. It's restricted to the document's HEAD, but you can use as many STYLE elements as you need.
<STYLE TYPE="text/css">
p { font-size: 12pt }
</STYLE>
The TYPE attribute defines style sheet language. For style properties for a given element, you can specify them directly with the element using the STYLE attribute, e.g.
<P STYLE="font-size: 12pt; color: rgb(0, 0, 100)">How do
you like this text style?
Properties defined in this way always apply, and have higher precedence than those defined in style sheets. The style sheet language can be set with the META element, e.g.
<META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css">
The default style sheet language can also be set by HTTP headers, e.g.
Content-Style-Type: text/css
If there are more than one such element or corresponding HTTP header, then the last one takes precedence over earlier ones. HTTP headers are considered as occuring earlier than the document HEAD for this purpose. In the absence of either a META element or an HTTP header, as a last resort, the default style sheet language type is assumed to be "text/css". It is recommended that authoring tools generate a META element to prevent this situation from occurring.
To define a cascade, you simply provide more than one LINK or STYLE elements, e.g.
<LINK REL=stylesheet HREF="corporate.css">
<LINK REL=stylesheet HREF="techreport.css">
<STYLE TYPE="text/css">
p.special { color: rgb(230, 100, 180) }
</STYLE>
The elements are cascaded in the order they appear in the HEAD. A cascade can include style sheets applicable to different media. The user agent is then responsible to filtering out those style sheets which are inapplicable to the current situation.
<LINK REL=stylesheet MEDIA=print HREF="corporate-print.css">
<LINK REL=stylesheet MEDIA=screen HREF="corporate-screen.css">
<LINK REL=stylesheet HREF="techreport.css">
<STYLE TYPE="text/css">
p.special { color: rgb(230, 100, 180) }
</STYLE>
Here, the author has defined a cascade where the "corporate" style sheet is provided in two forms, one suited to printing and the other for screen use. The "techreport" stylesheet and the rules defined by the STYLE element are for use in both situations. The MEDIA attribute can be used with both LINK and STYLE elements. Some of the permitted values include screen, print, aural and all. The complete list is defined later on in this specification.
Here is an example of using the MEDIA attribute with the STYLE element for adding sound effects to anchors for use in speech output:
<STYLE TYPE="text/css" MEDIA="aural">
A {cue-before: url(bell.aiff); cue-after: url(dong.wav)}
</STYLE>
LINK and STYLE elements without a TITLE attribute define persistent styles and are always applied, e.g.
<!-- a persistent style -->
<link href=xxx.css rel=stylesheet>
Adding a TITLE attribute converts the element to defining a default style, e.g.
<!-- a default style titled "Compact" -->
<link href=xxx.css title="Compact" rel=stylesheet>
Going yet further and adding the "alternate" keyword to the REL attribute (i.e. rel="alternate stylesheet") converts the element to defining an alternate style, e.g.
<!-- an alternate style titled "Medium" -->
<link href=xxx.css title="Medium" rel="alternate stylesheet">
<!-- an alternate style titled "Large" -->
<link href=xxx.css title="Large" rel="alternate stylesheet">
To define several alternate styles, you use the TITLE attribute to group LINK and STYLE elements belonging to the same style. All members of the same style must have exactly the same value for TITLE, but elements without the TITLE attribute are always applied (unless they are inappropriate to the current media).
<LINK REL="alternate stylesheet" TITLE="compact" HREF="small-base.css"> <LINK REL="alternate stylesheet" TITLE="compact" HREF="small-extras.css"> <LINK REL="alternate stylesheet" TITLE="big print" HREF="bigprint.css"> <LINK REL=stylesheet HREF="common.css">
Here, the author has provided two styles, one compact and one for increased legibility. The common style properties, which always apply, have been placed in a shared file "common.css". In addition, the compact style uses two files (small-base.css and small-extras.css), while the big-print style use just one (bigprint.css).
User agents should provide a means for users to view and pick from the list of alternative styles, and it is suggested that the value of the TITLE attribute be used to name each choice.
Sometimes its convenient to configure your Web server to specify the style sheet to be applied to a group of pages on your site. The HTTP Link header is taken to be the same as if a LINK element with the same attributes and values had occurred in the document's HEAD. Multiple Link headers correspond to multiple LINK elements occurring in the same order as the headers, e.g.
Link: REL=stylesheet HREF="corporate.css"
corresponds to:
<LINK REL=stylesheet HREF="corporate.css">
LINK and META elements implied by HTTP headers are defined as occurring before any explicit LINK and META elements in the document's HEAD.
You can specify several alternative styles, using multiple Link headers, and then use the REL attribute to determine the default style, e.g. in the following "compact" is applied by default since it omits the "alternate" keyword for the REL attribute:
Link: REL="stylesheet" TITLE="compact" HREF="compact.css" Link: REL="alternate stylesheet" TITLE="big print" HREF="bigprint.css"
This should also work when HTML documents are being by transported via email. Some email agents can alter the ordering of RFC 822 headers. To protect against this altering the cascading order for style sheets specified by Link headers, you can use header concatenation to merge several instances of the same header field. The quote marks are only needed when the attribute values include whitespace. Use SGML entities for character codes which are otherwise not permitted within HTTP or Email headers, or which are likely to effected by transit through gateways.
This specification builds upon the definition of the LINK element in HTML 2.0 (RFC 1866) in the following respects:
<!ENTITY % MEDIA
"(print|screen|projection|braille|aural|all)">
<!ELEMENT LINK - O EMPTY>
<!ATTLIST LINK
href CDATA #REQUIRED -- Uniform Resource Locator --
title CDATA #IMPLIED -- advisory title string --
rel CDATA #IMPLIED -- forward link relation --
rev CDATA #IMPLIED -- reverse link relation --
type CDATA #IMPLIED -- advisory Internet media type --
media %MEDIA ALL -- intended rendering medium --
>
<LINK REL=stylesheet MEDIA=print
HREF="printstylesheet.css" TYPE="text/css">
The MEDIA attribute is case insensitive. It takes one of the following values:
The MEDIA attribute can also take a comma-separated list of these values, e.g.
<LINK REL=stylesheet MEDIA="print, screen"
HREF="printstylesheet.css" TYPE="text/css">
In the future, there may be more values, and each value may associated with parameters. To enable such extensions to be introduced smoothly, user agents conforming to this specification must be able to parse the media type attribute value as follows:
media="screen, 3d-glasses, print and resolution > 90dpi"
is mapped to:
"screen"
"3d-glasses"
"print and resolution > 90dpi"
"screen"
"3d-glasses"
"print"
Note: style sheets may include media dependent variations within them. For instance the proposed CSS @media construct. In such cases it may be appropriate to use the default: media=all.
This allows authors to include style information as part of the document. One or more STYLE elements may be included in the document HEAD. It allows authors to provide style information as part of the document. The STYLE element requires a start and an end tag, e.g.
<HEAD>
<TITLE>Title</TITLE>
<STYLE TYPE="text/css">
H1 { color: black }
P { color: blue }
</STYLE>
</HEAD>
The STYLE element is formally defined by:
<!ELEMENT style - - CDATA>
<!ATTLIST style
type CDATA #IMPLIED -- Internet media type for style --
title CDATA #IMPLIED -- advisory title for this style --
media %MEDIA ALL -- intended rendering medium --
>
The attributes are defined as follows:
The content model for the STYLE element is defined as CDATA. In this kind of element, only one delimiter is recognized by a conforming parser: the end tag open (ETAGO) delimiter (i.e. the string "</"). The recognition of this delimiter is constrained to occur only when immediately followed by an SGML name start character ([a-zA-Z]). All characters which occur between the STYLE start tag and the first occurrence of ETAGO in such a context must be provided to the appropriate style sheet mechanism.
Note that all other SGML markup (such as comments, marked sections, etc.) appearing inside a STYLE element are construed to be actual character content of the STYLE element and not parsed as markup. A particular style sheet language may choose to treat such markup as it wishes; however such treatment should be documented.
Authors may wish to design their HTML documents to be viewable on older browsers that don't recognise the STYLE element. Unfortunately any style statements placed within a STYLE element will be visible to users. Some style sheet languages such as CSS allow the style statements to be enclosed in an SGML comment, for instance:
<STYLE TYPE="text/css">
<!--
H1 { color: red }
P {color: blue }
-->
</STYLE>
Note that for user agents conforming to this specification, the comment is interpreted by the application and not the SGML parser.
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 % style
"id ID #IMPLIED -- document wide unique id --
class CDATA #IMPLIED -- comma list of class values --
style CDATA #IMPLIED -- associated style info --">
For example:
<P CLASS="warning">Turn off the electrical power before unscrewing this plate. Failure to do so may result in a severe electrical shock.
CLASS values are case insensitive: Characters with multiple possible representations in ISO 10646 (e.g. both precomposed and base+diacritic forms) match only if they have the same representation, except for case differences, in both strings. Case folding must be performed as specified in the Unicode Standard, Version 2.0, section 4.1; in particular it is recommended that case-insensitive matching be peformed by folding to uppercase letters to lowercase, not vice versa. This is the same definition as used by XML.
For example:
<TITLE>Test Document</TITLE> <META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css"> <P STYLE="color: red; font-variant: small-caps">This text should be in small capitals and colored red!
<!ELEMENT span - - (%text)*>
<!ATTLIST span
%style; -- id, class, style --
>
Sometimes it is desirable to apply a style to some text which doesn't have a structural role or established rendering convention. For instance, the first few words of an article may be rendered as small capital letters ("small-caps"). In such situations it is inappropriate to use an existing tag such as EM: on existing user agents the first few words would be mysteriously italicized. The new SPAN tag is recommended instead, as it has no effect on existing user agents.
An example based on CSS:
<TITLE>Title</TITLE>
<STYLE TYPE="text/css">
SPAN { font-variant: small-caps }
</STYLE>
<P><SPAN>The first</SPAN> few words of
this article are in small-caps.
This would be formatted to look something like:
THE FIRST few words of this
article are in small-caps.
While on an existing user agent it would look like:
The first few words of this
article is small-caps.
The SPAN element can be used anywhere that the HTML EM element is allowed.
<!ELEMENT div - - (%body.content)*>
<!ATTLIST div
%style; -- id, class, style --
>
The DIV element is used with the CLASS attribute to represent different kinds of containers, e.g. chapter, section, abstract, or appendix. DIV allows the enclosed group of elements to be given a distinctive style. For example:
<DIV CLASS=Abstract>
<P>The Chieftain product range is our market winner for
the coming year. This report sets out how to position
Chieftain against competing products.
<P>Chieftain replaces the Commander range, which will
remain on the price list until further notice.
</DIV>
The DIV element can be used anywhere that the HTML P element is allowed. The content model for DIV allows headers, lists, paragraphs as well as other DIV elements etc. This allows DIVs to be nested, to form hierarchies of chapters, sections, and subsections etc.
In an interactive user agent, the user should be made aware that a particular style sheet has been applied, and be given the option of turning it off or selecting a different style. A flag in the corner of the window and a toggle in a menubar can be sufficient.
It may be possible for the user to combine several of the available style sheets. It should be possible for the user to choose a personal style instead of, or in combination with, style sheets supplied by the author.
Some people have voiced concerns over performance issues for style sheets. For instance, fetching a LINKed style sheet may delay the full presentation for the user. 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 style rules throughout 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.
W3C is working on a new version of HTML with the features described in this specification. See http://www.w3.org/pub/WWW/MarkUp for the current status of this work.
| Last update: | 28th | June, | 1998 |