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<h1 align="center"><a name="h-14">14</a> <a name="stylesheets">Style
Sheets</a></h1>
<div class="subtoc">
<p><strong>Contents</strong></p>
<ol>
<li><a class="tocxref" href="#h-14.1">Introduction to style sheets</a></li>
<li><a class="tocxref" href="#h-14.2">Adding style to HTML</a>
<ol>
<li><a class="tocxref" href="#h-14.2.1">Setting the default style sheet
language</a></li>
<li><a class="tocxref" href="#h-14.2.2">Inline style information</a></li>
<li><a class="tocxref" href="#h-14.2.3">Header style information: the <samp class="einst2">STYLE</samp> element</a></li>
<li><a class="tocxref" href="#h-14.2.4">Media types</a></li>
</ol>
</li>
<li><a class="tocxref" href="#h-14.3">External style sheets</a>
<ol>
<li><a class="tocxref" href="#h-14.3.1">Preferred and alternate style
sheets</a></li>
<li><a class="tocxref" href="#h-14.3.2">Specifying external style
sheets</a></li>
</ol>
</li>
<li><a class="tocxref" href="#h-14.4">Cascading style sheets</a>
<ol>
<li><a class="tocxref" href="#h-14.4.1">Media-dependent cascades</a></li>
<li><a class="tocxref" href="#h-14.4.2">Inheritance and cascading</a></li>
</ol>
</li>
<li><a class="tocxref" href="#h-14.5">Hiding style data from user
agents</a></li>
<li><a class="tocxref" href="#h-14.6">Linking to style sheets with HTTP
headers</a></li>
</ol>
</div>
<h2><a name="h-14.1">14.1</a> <span class="index-inst" title="style
sheet::introduction to"><a name="idx-style_sheet-3">Introduction to style
sheets</a></span></h2>
<p>Style sheets represent a major breakthrough for Web page designers,
expanding their ability to improve the appearance of their pages. In the
scientific environments in which the Web was conceived, people are more
concerned with the content of their documents than the presentation. As people
from wider walks of life discovered the Web, the limitations of HTML became a
source of continuing frustration and authors were forced to sidestep HTML's
stylistic limitations. While the intentions have been good -- to improve the
presentation of Web pages -- the techniques for doing so have had unfortunate
side effects. These techniques work for some of the people, some of the time,
but not for all of the people, all of the time. They include:</p>
<ul>
<li>Using proprietary HTML extensions</li>
<li>Converting text into images</li>
<li>Using images for white space control</li>
<li>Use of tables for page layout</li>
<li>Writing a program instead of using HTML</li>
</ul>
<p>These techniques considerably increase the complexity of Web pages, offer
limited flexibility, suffer from interoperability problems, and create
hardships for people with disabilities.</p>
<p>Style sheets solve these problems at the same time they supersede the
limited range of presentation mechanisms in HTML. Style sheets make it easy to
specify the amount of white space between text lines, the amount lines are
indented, the colors used for the text and the backgrounds, the font size and
style, and a host of other details.</p>
<div class="example">
<p>For example, the following short CSS style sheet (stored in the file
"special.css"), sets the text color of a paragraph to green and surrounds it
with a solid red border:</p>
<pre>P.special {
color : green;
border: solid red;
}
</pre>
<p>Authors may link this style sheet to their source HTML document with the <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> element:</p>
<pre><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
<HTML>
<HEAD>
</HEAD>
<BODY>
<P class="special">This paragraph should have special green text.
</BODY>
</HTML>
</pre>
</div>
<p>HTML 4 provides support for the following style sheet features:</p>
<dl>
<dt><b>Flexible placement of style information</b></dt>
<dd>Placing style sheets in separate files makes them easy to reuse. Sometimes
it's useful to include rendering instructions within the document to which they
apply, either grouped at the start of the document, or in attributes of the
elements throughout the body of the document. To make it easier to manage style
on a site basis, this specification describes how to use HTTP headers to set
the style sheets to be applied to a document.</dd>
<dt><b>Independence from specific style sheet languages</b></dt>
<dd>This specification doesn't tie HTML to any particular style sheet language.
This allows for a range of such languages to be used, for instance simple ones
for the majority of users and much more complex ones for the minority of users
with highly specialized needs. The examples included below all use the CSS
(Cascading Style Sheets) language <a rel="biblioentry" href="http://www.w3.org/TR/html401/references.html#ref-CSS1" class="informref">[CSS1]</a>, but other style
sheet languages would be possible.</dd>
<dt><b>Cascading</b></dt>
<dd>This is the capability provided by some style sheet languages such as CSS
to allow style information from several sources to be blended together. These
could be, for instance, corporate style guidelines, styles common to a group of
documents, and styles specific to a single document. By storing these
separately, style sheets can be reused, simplifying authoring and making more
effective use of network caching. The cascade defines an ordered sequence of
style sheets where rules in later sheets have greater precedence than earlier
ones. Not all style sheet languages support cascading.</dd>
<dt><span class="index-inst" title="accessibility::and style sheets"><a name="idx-accessibility"><b>Media dependencies</b></a></span></dt>
<dd>HTML allows authors to specify documents in a media-independent way. This
allows users to access Web pages using a wide variety of devices and media,
e.g., graphical displays for computers running Windows, Macintosh OS, and X11,
devices for television sets, specially adapted phones and PDA-based portable
devices, speech-based browsers, and braille-based tactile devices.
<p>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.</p>
</dd>
<dt><b>Alternate styles</b></dt>
<dd>Authors may wish to offer readers several ways to view a document. For
instance, a style sheet for rendering compact documents with small fonts, or
one that specifies larger fonts for increased legibility. This specification
allows authors to specify a preferred style sheet as well as alternates that
target specific users or media. User agents should give users the opportunity
to select from among alternate style sheets or to switch off style sheets
altogether.</dd>
<dt><b>Performance concerns</b></dt>
<dd>Some people have voiced concerns over performance issues for style sheets.
For instance, retrieving an external 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.
<p>The current proposal addresses 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.</p>
<p>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.</p>
</dd>
</dl>
<h2><a name="h-14.2">14.2</a> Adding style to HTML</h2>
<div class="note">
<p><em><strong>Note.</strong> The sample default style sheet for HTML 4 that is
included in <a rel="biblioentry" href="http://www.w3.org/TR/html401/references.html#ref-CSS2" class="informref">[CSS2]</a> expresses generally accepted default style information
for each element. Authors and implementors alike might find this a useful
resource.</em></p>
</div>
<p>HTML documents may contain style sheet rules directly in them or they may
import style sheets.</p>
<p>Any style sheet language may be used with HTML. A simple style sheet
language may suffice for the needs of most users, but other languages may be
more suited to highly specialized needs. This specification uses the style
language "Cascading Style Sheets" (<a rel="biblioentry" href="http://www.w3.org/TR/html401/references.html#ref-CSS1" class="informref">[CSS1]</a>), abbreviated CSS,
for examples.</p>
<p>The <a href="http://www.w3.org/TR/html401/types.html#type-style">syntax of style data</a> depends on
the style sheet language.</p>
<h3><a name="h-14.2.1">14.2.1</a> <span class="index-inst" title="default::style sheet language|style sheet
language::default|HTTP::Content-Style-Type header|Content-Style-Type header"><a name="default-style">Setting the default style sheet language</a></span></h3>
<p>Authors must specify the style sheet language of style information
associated with an HTML document.</p>
<p>Authors should use the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">META</samp></a> element to set the default style
sheet language for a document. For example, to set the default to CSS, authors
should put the following declaration in the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-HEAD" class="noxref"><samp class="einst">
HEAD</samp></a> of their documents:</p>
</pre>
<p>The default style sheet language may also be set with HTTP headers. The
above <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">META</samp></a> declaration is equivalent to the HTTP header:</p>
</pre>
<p>User agents should determine the default style sheet language for a document
according to the following steps (highest to lowest priority):</p>
<ol>
<li>If any <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">META</samp></a> declarations specify the "Content-Style-Type",
the last one in the character stream determines the default style sheet
language.</li>
<li>Otherwise, if any HTTP headers specify the "Content-Style-Type", the last
one in the character stream determines the default style sheet language.</li>
</ol>
<p>Documents that include elements that set the <a href="http://www.w3.org/TR/html401/present/styles.html#adef-style" class="noxref"><samp class="ainst">style</samp></a>
attribute but which don't define a default style sheet language are incorrect.
<span class="index-inst" title="authoring tool::and default style sheet
language"><a name="idx-authoring_tool">Authoring tools</a></span> should
generate default style sheet language information (typically a <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">
META</samp></a> declaration) so that user agents do not have to rely on a
<h3><a name="h-14.2.2">14.2.2</a> <a name="style-element">Inline style
information</a></h3>
<div class="adef-list">
<p><em>Attribute definitions</em></p>
<dl>
<dt><a name="adef-style"><samp class="adef">style</samp></a> = <a href="http://www.w3.org/TR/html401/types.html#type-style"><em>style</em></a> <a href="http://www.w3.org/TR/html401/types.html#case-neutral">[CN]</a></dt>
<dd>This attribute specifies style information for the current element.</dd>
</dl>
</div>
<p><span class="index-inst" title="style sheet::inline rules"><a name="idx-style_sheet-4">The</a></span> <a href="http://www.w3.org/TR/html401/types.html#type-style">
syntax</a> of the value of the <a href="http://www.w3.org/TR/html401/present/styles.html#adef-style" class="noxref">
<samp class="ainst">style</samp></a> attribute is determined by the <a href="#default-style">default style sheet language</a>. For example, for [[CSS2]]
inline style, use the declaration block syntax described in section 4.1.8
(without curly brace delimiters).</p>
<div class="example">
<p>This CSS example sets color and font size information for the text in a
specific paragraph.</p>
<pre><P style="font-size: 12pt; color: fuchsia">Aren't style sheets wonderful?
</pre>
<p>In CSS, property declarations have the form "name : value" and are separated
by a semi-colon.</p>
</div>
<p>To specify style information for more than one element, authors should use
the <samp class="edef">STYLE</samp> element. For optimal flexibility, authors
should define styles in external style sheets.</p>
<h3><a name="h-14.2.3">14.2.3</a> <a name="style-group">Header style
information</a>: the <a name="edef-STYLE"><samp class="edef">STYLE</samp></a>
element</h3>
<div class="dtd-fragment">
<pre class="dtd-fragment"><!ELEMENT <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a> - - <a href="http://www.w3.org/TR/html401/sgml/dtd.html#StyleSheet">%StyleSheet</a> -- style info -->
<!ATTLIST STYLE
<a href="http://www.w3.org/TR/html401/sgml/dtd.html#i18n">%i18n;</a> -- <a href="http://www.w3.org/TR/html401/struct/dirlang.html#adef-lang">lang</a>, <a href="http://www.w3.org/TR/html401/struct/dirlang.html#adef-dir">dir</a>, for use with title --
<a href="http://www.w3.org/TR/html401/present/styles.html#adef-type-STYLE" class="noxref"><samp class="ainst-STYLE">type</samp></a> <a href="http://www.w3.org/TR/html401/sgml/dtd.html#ContentType">%ContentType;</a> #REQUIRED -- content type of style language --
<a href="http://www.w3.org/TR/html401/present/styles.html#adef-media" class="noxref"><samp class="ainst-STYLE">media</samp></a> <a href="http://www.w3.org/TR/html401/sgml/dtd.html#MediaDesc">%MediaDesc;</a> #IMPLIED -- designed for use with these media --
<a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst-STYLE">title</samp></a> <a href="http://www.w3.org/TR/html401/sgml/dtd.html#Text">%Text;</a> #IMPLIED -- advisory title --
>
</pre>
</div>
<p><em>Start tag: <strong>required</strong>, End tag: <strong>
required</strong></em></p>
<div class="adef-list">
<p><em>Attribute definitions</em></p>
<dl>
<dt><a name="adef-type-STYLE"><samp class="adef">type</samp></a> = <a href="http://www.w3.org/TR/html401/types.html#type-content-type"><em>content-type</em></a> <a href="http://www.w3.org/TR/html401/types.html#case-insensitive">[CI]</a></dt>
<dd>This attribute specifies the style sheet language of the element's contents
and overrides the default style sheet language. The style sheet language is
this attribute; there is no default value for this attribute.</dd>
<dt><a name="adef-media"><samp class="adef">media</samp></a> = <a href="http://www.w3.org/TR/html401/types.html#type-media-descriptors"><em>media-descriptors</em></a> <a href="http://www.w3.org/TR/html401/types.html#case-insensitive">[CI]</a></dt>
<dd>This attribute specifies the intended destination medium for style
information. It may be a single media descriptor or a comma-separated list. The
default value for this attribute is "screen".</dd>
</dl>
</div>
<div class="aref-list">
<p><em>Attributes defined elsewhere</em></p>
<ul>
<li><a href="http://www.w3.org/TR/html401/struct/dirlang.html#adef-lang" class="noxref"><samp class="ainst">lang</samp></a> (<a href="http://www.w3.org/TR/html401/struct/dirlang.html#language-info">language information</a>), <a href="http://www.w3.org/TR/html401/struct/dirlang.html#adef-dir" class="noxref"><samp class="ainst">
direction</a>)</li>
<li><a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst">title</samp></a> (<a href="http://www.w3.org/TR/html401/struct/global.html#title">element
title</a>)</li>
</ul>
</div>
<p>The <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">
STYLE</samp></a> element allows authors to put style sheet rules in the <span class="index-inst" title="style sheet::rules in HEAD"><a name="idx-style_sheet-5">head</a></span> of the document. HTML permits any number of
<a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">
STYLE</samp></a> elements in the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-HEAD" class="noxref"><samp class="einst">HEAD</samp></a> section of a document.</p>
<p>User agents that don't support style sheets, or don't support the specific
style sheet language used by a <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref">
<samp class="einst">STYLE</samp></a> element, must hide the contents of the <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">
STYLE</samp></a> element. <span class="index-inst" title="error::rendering
style rules in STYLE"><a name="idx-error">It is an error</a></span> to render
the content as part of the document's text. Some style sheet languages support
syntax for <a href="#hiding">hiding the content</a> from non-conforming user
agents.</p>
<p>The <a href="http://www.w3.org/TR/html401/types.html#type-style">syntax of style data</a> depends on
the style sheet language.</p>
<p>Some style sheet implementations may allow a wider variety of rules in the
<a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">
STYLE</samp></a> element than in the <a href="http://www.w3.org/TR/html401/present/styles.html#adef-style" class="noxref"><samp class="ainst">style</samp></a> attribute. For example, with CSS,
rules may be declared within a <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref">
<samp class="einst">STYLE</samp></a> element for:</p>
<ul>
<li>All instances of a specific HTML element (e.g., all <a href="http://www.w3.org/TR/html401/struct/text.html#edef-P" class="noxref"><samp class="einst">P</samp></a>
elements, all <a href="http://www.w3.org/TR/html401/struct/global.html#edef-H1" class="noxref"><samp class="einst">H1</samp></a> elements, etc.)</li>
<li>All instances of an HTML element belonging to a specific class (i.e., whose
<a href="http://www.w3.org/TR/html401/struct/global.html#adef-class" class="noxref"><samp class="ainst">
class</samp></a> attribute is set to some value).</li>
<li>Single instances of an HTML element (i.e., whose <a href="http://www.w3.org/TR/html401/struct/global.html#adef-id" class="noxref"><samp class="ainst">
id</samp></a> attribute is set to some value).</li>
</ul>
<p>Rules for style rule precedences and inheritance depend on the style sheet
language.</p>
<div class="example">
<p>The following CSS <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a> declaration puts a border around every <a href="http://www.w3.org/TR/html401/struct/global.html#edef-H1" class="noxref"><samp class="einst">
H1</samp></a> element in the document and centers it on the page.</p>
<pre><HEAD>
H1 {border-width: 1; border: solid; text-align: center}
</STYLE>
</HEAD>
</pre>
<p>To specify that this style information should only apply to <a href="http://www.w3.org/TR/html401/struct/global.html#edef-H1" class="noxref"><samp class="einst">
H1</samp></a> elements of a specific class, we modify it as follows:</p>
<pre><HEAD>
H1.myclass {border-width: 1; border: solid; text-align: center}
</STYLE>
</HEAD>
<BODY>
<H1 class="myclass"> This H1 is affected by our style </H1>
<H1> This one is not affected by our style </H1>
</BODY>
</pre>
<p>Finally, to limit the scope of the style information to a single instance of
<a href="http://www.w3.org/TR/html401/struct/global.html#edef-H1" class="noxref"><samp class="einst">
H1</samp></a>, set the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-id" class="noxref">
<samp class="ainst">id</samp></a> attribute:</p>
<pre><HEAD>
#myid {border-width: 1; border: solid; text-align: center}
</STYLE>
</HEAD>
<BODY>
<H1 class="myclass"> This H1 is not affected </H1>
<H1 id="myid"> This H1 is affected by style </H1>
<H1> This H1 is not affected </H1>
</BODY>
</pre>
</div>
<p><span class="index-inst" title="style sheet::used with DIV and SPAN"><a name="idx-style_sheet-6">Although style information</a></span> may be set for
almost every HTML element, two elements, <a href="http://www.w3.org/TR/html401/struct/global.html#edef-DIV" class="noxref"><samp class="einst">
DIV</samp></a> and <a href="http://www.w3.org/TR/html401/struct/global.html#edef-SPAN" class="noxref">
<samp class="einst">SPAN</samp></a>, are particularly useful in that they do
not impose any presentation semantics (besides <a href="http://www.w3.org/TR/html401/struct/global.html#block-inline">block-level vs. inline</a>). When combined
with style sheets, these elements allow users to extend HTML indefinitely,
particularly when used with the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-class" class="noxref"><samp class="ainst">class</samp></a> and <a href="http://www.w3.org/TR/html401/struct/global.html#adef-id" class="noxref"><samp class="ainst">
id</samp></a> attributes.</p>
<div class="example">
<p>In the following example, we use the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-SPAN" class="noxref"><samp class="einst">
SPAN</samp></a> element to set the font style of the first few words of a
paragraph to small caps.</p>
<pre><HEAD>
SPAN.sc-ex { font-variant: small-caps }
</STYLE>
</HEAD>
<BODY>
<P><SPAN class="sc-ex">The first</SPAN> few words of
this paragraph are in small-caps.
</BODY>
</pre>
</div>
<div class="example">
<p>In the following example, we use <a href="http://www.w3.org/TR/html401/struct/global.html#edef-DIV" class="noxref"><samp class="einst">DIV</samp></a> and the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-class" class="noxref"><samp class="ainst">
class</samp></a> attribute to set the text justification for a series of
paragraphs that make up the abstract section of a scientific article. This
style information could be reused for other abstract sections by setting the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-class" class="noxref"><samp class="ainst">
class</samp></a> attribute elsewhere in the document.</p>
<pre><HEAD>
DIV.Abstract { text-align: justify }
</STYLE>
</HEAD>
<BODY>
<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>
</BODY>
</pre>
</div>
<h3><a name="h-14.2.4">14.2.4</a> <span class="index-inst" title="media::used
with style sheets|style sheet::target media for"><a name="media-types">Media
types</a></span></h3>
<p>HTML allows authors to design documents that take advantage of the
characteristics of the media where the document is to be rendered (e.g.,
graphical displays, television screens, handheld devices, speech-based
browsers, braille-based tactile devices, etc.). By specifying the <a href="http://www.w3.org/TR/html401/present/styles.html#adef-media" class="noxref"><samp class="ainst">media</samp></a>
attribute, authors allow user agents to load and apply style sheets
selectively. Please consult the list of recognized <a href="http://www.w3.org/TR/html401/types.html#type-media-descriptors">media descriptors</a>.</p>
<div class="example">
<p>The following sample declarations apply to <a href="http://www.w3.org/TR/html401/struct/global.html#edef-H1" class="noxref"><samp class="einst">
H1</samp></a> elements. When projected in a business meeting, all instances
will be blue. When printed, all instances will be centered.</p>
<pre><HEAD>
H1 { color: blue}
</STYLE>
H1 { text-align: center }
</STYLE>
</pre>
<p>This example adds sound effects to anchors for use in speech output:</p>
</STYLE>
</HEAD>
</pre>
</div>
<p>Media control is particularly interesting when applied to external style
sheets since user agents can save time by retrieving from the network only
those style sheets that apply to the current device. For instance, speech-based
browsers can avoid downloading style sheets designed for visual rendering. See
the section on <a href="#media-cascades">media-dependent cascades</a> for more
information.</p>
<h2><a name="h-14.3">14.3</a> <span class="index-inst" title="style
sheet::external"><a name="style-external">External style sheets</a></span></h2>
<p>Authors may separate style sheets from HTML documents. This offers several
benefits:</p>
<ul>
<li>Authors and Web site managers may share style sheets across a number of
documents (and sites).</li>
<li>Authors may change the style sheet without requiring modifications to the
document.</li>
<li>User agents may load style sheets selectively (based on media
descriptions).</li>
</ul>
<h3><a name="h-14.3.1">14.3.1</a> Preferred and alternate style sheets</h3>
<p>HTML allows authors to associate any number of external style sheets with a
document. The style sheet language defines how multiple external style sheets
interact (for example, the CSS "cascade" rules).</p>
<p><span class="index-def" title="style sheet::alternate|alternate style
sheets"><a name="didx-style_sheet">Authors may specify a number of mutually
exclusive style sheets called <dfn>alternate</dfn> style sheets.</a></span>
Users may select their favorite among these depending on their preferences. For
instance, an author may specify one style sheet designed for small screens and
another for users with weak vision (e.g., large fonts). User agents should
allow users to select from alternate style sheets.</p>
<p><span class="index-def" title="style sheet::preferred|preferred style
sheets"><a name="didx-style_sheet-1">The author may specify that one of the
alternates is a <dfn>preferred</dfn> style sheet.</a></span> User agents should
apply the author's preferred style sheet unless the user has selected a
different alternate.</p>
<p>Authors may group several alternate style sheets (including the author's
preferred style sheets) under a single <dfn>style name</dfn>. When a user
selects a named style, the user agent must apply all style sheets with that
name. User agents must not apply alternate style sheets with a different style
name. The section on <a href="#specifying-external">specifying external style
sheets</a> explains how to name a group of style sheets.</p>
<p><span class="index-def" title="style sheet::persistent|persistent style
sheets"><a name="didx-style_sheet-2">Authors may also specify <dfn>
persistent</dfn> style sheets that user agents must apply in addition to any
alternate style sheet.</a></span></p>
<p>User agents must respect <a href="http://www.w3.org/TR/html401/types.html#type-media-descriptors">
media descriptors</a> when applying any style sheet.</p>
<p>User agents should also allow users to disable the author's style sheets
entirely, in which case the user agent must not apply any persistent or
alternate style sheets.</p>
<h3><a name="h-14.3.2">14.3.2</a> <span class="index-inst" title="style
sheet::specification of external|link::and external style sheets"><a name="specifying-external">Specifying external style sheets</a></span></h3>
<p>Authors specify external style sheets with the following attributes of the
<a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> element:</p>
<ul>
<li>Set the value of <a href="http://www.w3.org/TR/html401/struct/links.html#adef-href" class="noxref">
<samp class="ainst">href</samp></a> to the location of the style sheet file.
The value of <a href="http://www.w3.org/TR/html401/struct/links.html#adef-href" class="noxref"><samp class="ainst">href</samp></a> is a <a href="http://www.w3.org/TR/html401/types.html#type-uri">
URI</a>.</li>
<li>Set the value of the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-type-A" class="noxref"><samp class="ainst-A">type</samp></a> attribute to indicate the
language of the linked (style sheet) resource. This allows the user agent to
avoid downloading a style sheet for an unsupported style sheet language.</li>
<li>Specify that the style sheet is persistent, preferred, or alternate:
<ul>
<li>To make a style sheet persistent, set the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" class="noxref"><samp class="ainst">
rel</samp></a> attribute to "stylesheet" and don't set the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst">
title</samp></a> attribute.</li>
<li>To make a style sheet preferred, set the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" class="noxref"><samp class="ainst">
rel</samp></a> attribute to "stylesheet" and name the style sheet with the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst">
title</samp></a> attribute.</li>
<li>To specify an alternate style sheet, set the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" class="noxref"><samp class="ainst">
rel</samp></a> attribute to "alternate stylesheet" and name the style sheet
with the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst">title</samp></a> attribute.</li>
</ul>
</li>
</ul>
<p>User agents should provide a means for users to view and pick from the list
of alternate styles. The value of the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst">
title</samp></a> attribute is recommended as the name of each choice.</p>
<div class="example">
<p>In this example, we first specify a persistent style sheet located in the
</pre>
<p>Setting the <a href="http://www.w3.org/TR/html401/struct/global.html#adef-title" class="noxref"><samp class="ainst">title</samp></a> attribute makes this the author's preferred
style sheet:</p>
</pre>
<p>Adding the keyword "alternate" to the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" class="noxref"><samp class="ainst">
rel</samp></a> attribute makes it an alternate style sheet:</p>
</pre>
</div>
<p>For more information on external style sheets, please consult the section on
<a href="http://www.w3.org/TR/html401/struct/links.html#linksandss">links and external style
sheets.</a></p>
<p><span class="index-inst" title="HTTP::Default-Style header|style
sheet::specification of preferred">Authors may also use the <a name="idx-HTTP-1" href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">META</samp></a> element to set the document's preferred style
sheet.</span> For example, to set the preferred style sheet to "compact" (see
the preceding example), authors may include the following line in the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-HEAD" class="noxref"><samp class="einst">
HEAD</samp></a>:</p>
<div class="example">
<pre><META http-equiv="Default-Style" content="compact">
</pre>
</div>
<p>The preferred style sheet may also be specified with HTTP headers. The above
<a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">
META</samp></a> declaration is equivalent to the HTTP header:</p>
<div class="example">
<pre>Default-Style: "compact"
</pre>
</div>
<p>If two or more <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref">
<samp class="einst">META</samp></a> declarations or HTTP headers specify the
preferred style sheet, the last one takes precedence. HTTP headers are
considered to occur earlier than the document <a href="http://www.w3.org/TR/html401/struct/global.html#edef-HEAD" class="noxref"><samp class="einst">
HEAD</samp></a> for this purpose.</p>
<p>If two or more <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">LINK</samp></a> elements specify a preferred style sheet, the
first one takes precedence.</p>
<p>Preferred style sheets specified with <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref"><samp class="einst">
META</samp></a> or HTTP headers have precedence over those specified with the
<a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> element.</p>
<h2><a name="h-14.4">14.4</a> <span class="index-inst" title="style
sheet::cascading|cascading style sheets"><a name="idx-style_sheet-11">Cascading
style sheets</a></span></h2>
<p><dfn>Cascading</dfn> style sheet languages such as CSS allow style
information from several sources to be blended together. However, not all style
sheet languages support cascading. To define a cascade, authors specify a
sequence of <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">LINK</samp></a> and/or <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a> elements. The style information
is cascaded in the order the elements appear in the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-HEAD" class="noxref"><samp class="einst">
HEAD</samp></a>.</p>
<div class="note">
<p><em><strong>Note.</strong> This specification does not specify how style
sheets from different style languages cascade. Authors should avoid mixing
style sheet languages.</em></p>
</div>
<div class="example">
<p>In the following example, we specify two alternate style sheets named
"compact". If the user selects the "compact" style, the user agent must apply
both external style sheets, as well as the persistent "common.css" style sheet.
If the user selects the "big print" style, only the alternate style sheet
</pre>
</div>
<div class="example">
<p>Here is a cascade example that involves both the <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> and <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a> elements.</p>
p.special { color: rgb(230, 100, 180) }
</STYLE>
</pre>
</div>
<h3><a name="h-14.4.1">14.4.1</a> <span class="index-inst" title="media::and
external style sheets|link::and media-dependent style sheets"><a name="media-cascades">Media-dependent cascades</a></span></h3>
<p>A cascade may include style sheets applicable to different media. Both <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> and <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a> may be used with the <a href="http://www.w3.org/TR/html401/present/styles.html#adef-media" class="noxref"><samp class="ainst">media</samp></a>
attribute. The user agent is then responsible for filtering out those style
sheets that do not apply to the current medium.</p>
<div class="example">
<p>In the following example, we define a cascade where the "corporate" style
sheet is provided in several versions: one suited to printing, one for screen
use and one for speech-based browsers (useful, say, when reading email in the
car). The "techreport" stylesheet applies to all media. The color rule defined
by the <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">
STYLE</samp></a> element is used for print and screen but not for aural
rendering.</p>
p.special { color: rgb(230, 100, 180) }
</STYLE>
</pre>
</div>
<h3><a name="h-14.4.2">14.4.2</a> Inheritance and cascading</h3>
<p>When the user agent wants to render a document, it needs to find values for
style properties, e.g. the font family, font style, size, line height, text
color and so on. The exact mechanism depends on the style sheet language, but
the following description is generally applicable:</p>
<p>The cascading mechanism is used when a number of style rules all apply
directly to an element. The mechanism allows the user agent to sort the rules
by specificity, to determine which rule to apply. If no rule can be found, the
next step depends on whether the style property can be inherited or not. Not
all properties can be inherited. For these properties the style sheet language
provides default values for use when there are no explicit rules for a
particular element.</p>
<p>If the property can be inherited, the user agent examines the immediately
enclosing element to see if a rule applies to that. This process continues
until an applicable rule is found. This mechanism allows style sheets to be
specified compactly. For instance, authors may specify the font family for all
elements within the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-BODY" class="noxref">
<samp class="einst">BODY</samp></a> by a single rule that applies to the <a href="http://www.w3.org/TR/html401/struct/global.html#edef-BODY" class="noxref"><samp class="einst">
BODY</samp></a> element.</p>
<h2><a name="h-14.5">14.5</a> <span class="index-inst" title="user agent::and
style data|style sheet::comments to hide|comments::used to hide style sheet
data"><a name="hiding">Hiding style data from user agents</a></span></h2>
<p>Some style sheet languages support syntax intended to allow authors to hide
the content of <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a> elements from non-conforming user agents.</p>
<div class="example">
<p>This example illustrates for CSS how to comment out the content of <a href="http://www.w3.org/TR/html401/present/styles.html#edef-STYLE" class="noxref"><samp class="einst">STYLE</samp></a>
elements to ensure that older, non-conforming user agents will not render them
as text.</p>
<!--
H1 { color: red }
P { color: blue}
-->
</STYLE>
</pre>
</div>
<h2><a name="h-14.6">14.6</a> <span class="index-inst" title="HTTP::used to
link external style sheets"><a name="idx-HTTP-2">Linking to style sheets with
HTTP headers</a></span></h2>
<p><em>This section only applies to user agents conforming to versions of HTTP
that define a Link header field. Note that HTTP 1.1 as defined by <a rel="biblioentry" href="http://www.w3.org/TR/html401/references.html#ref-RFC2616" class="informref">
[RFC2616]</a> does not include a Link header field (refer to section
19.6.3).</em></p>
<p>Web server managers may find it convenient to configure a server so that a
style sheet will be applied to a group of pages. The HTTP <tt>Link</tt> header
has the same effect as a <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">LINK</samp></a> element with the same attributes
and values. Multiple <tt>Link</tt> headers correspond to multiple <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> elements occurring in the same order. For instance,</p>
<pre>Link: <http://www.acme.com/corporate.css>; REL=stylesheet
</pre>
<p>corresponds to:</p>
<pre><LINK rel="stylesheet" href="http://www.acme.com/corporate.css">
</pre>
<p>It is possible to specify several alternate styles using multiple <tt>
Link</tt> headers, and then use the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" class="noxref"><samp class="ainst">rel</samp></a> attribute to determine the
default style.</p>
<div>
<p>In the following example, "compact" is applied by default since it omits the
"alternate" keyword for the <a href="http://www.w3.org/TR/html401/struct/links.html#adef-rel" class="noxref"><samp class="ainst">rel</samp></a> attribute.</p>
<pre>Link: <compact.css>; rel="stylesheet"; title="compact"
Link: <bigprint.css>; rel="alternate stylesheet"; title="big print"
</pre>
</div>
<p>This should also work when HTML documents are sent by email. Some email
agents can alter the ordering of <a rel="biblioentry" href="http://www.w3.org/TR/html401/references.html#ref-RFC822" class="informref">[RFC822]</a> headers. To
protect against this affecting the cascading order for style sheets specified
by <tt>Link</tt> headers, authors 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 to reference characters
that are otherwise not permitted within HTTP or email headers, or that are
likely to be affected by transit through gateways.</p>
<p><a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> and <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref">
<samp class="einst">META</samp></a> elements implied by HTTP headers are
defined as occurring before any explicit <a href="http://www.w3.org/TR/html401/struct/links.html#edef-LINK" class="noxref"><samp class="einst">
LINK</samp></a> and <a href="http://www.w3.org/TR/html401/struct/global.html#edef-META" class="noxref">
<samp class="einst">META</samp></a> elements in the document's <a href="http://www.w3.org/TR/html401/struct/global.html#edef-HEAD" class="noxref"><samp class="einst">
HEAD</samp></a>.</p>
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