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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<html lang="en">
<!-- $Id: intro.src,v 2.52 2010-08-06 19:42:21 bbos Exp $ -->
<HEAD>
<TITLE>Introduction to CSS 2.1</TITLE>
</HEAD>
<BODY>
<H1>Introduction to CSS 2.1</H1>
<H2><a name="html-tutorial">A brief CSS 2.1 tutorial for HTML</a></H2>
<p>This section is non-normative.
<P> In this tutorial, we show how easy it can be to design simple
style sheets. For this tutorial, you will need to know a little HTML
(see [[-HTML4]]) and some basic desktop publishing terminology.
<P>We begin with a small HTML document:</p>
<PRE class="html-example">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
</PRE>
<P>To set the text color of the H1 elements to red, you can write the
following CSS rules:</P>
<PRE class="example">
h1 { color: red }
</PRE>
<P>A CSS rule consists of two main parts: <a
href="selector.html">selector</a> ('h1') and declaration
('color: red'). In HTML, element names are case-insensitive so
'h1' works just as well as 'H1'. The declaration has two parts:
property name ('color') and property value ('red'). While the example above tries to
influence only one of the properties needed for rendering an HTML
document, it qualifies as a style sheet on its own. Combined with
other style sheets (one fundamental feature of CSS is that style
sheets are combined), the rule will determine the final presentation of the
document.
<P> The HTML 4 specification defines how style sheet rules may be
specified for HTML documents: either within the HTML document, or via
an external style sheet. To put the style sheet into the document, use
the STYLE element:</p>
<PRE class="html-example">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
h1 { color: red }
</STYLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
</PRE>
<P> For maximum flexibility, we recommend that authors specify
external style sheets; they may be changed without modifying the
source HTML document, and they may be shared among several
documents. To link to an external style sheet, you can use the LINK
element:</p>
<PRE class="html-example">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<LINK rel="stylesheet" href="bach.css" type="text/css">
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
</PRE>
<P>The LINK element specifies:</p>
<ul>
<li>the type of link: to a "stylesheet".
<li>the location of the style sheet via the "href" attribute.
<li>the type of style sheet being linked: "text/css".
</ul>
<P>To show the close relationship between a style sheet and the
structured markup, we continue to use the STYLE element in this
tutorial. Let's add more colors:
<PRE class="html-example">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
body { color: black; background: white }
h1 { color: red; background: white }
</STYLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
</PRE>
<P>The style sheet now contains four rules: the first two set the
color and background of the BODY element (it's a good idea to set the
text color and background color together), while the last two
set the color and the background of the H1 element. Since no color
has been specified for the P element, it will inherit the color
from its parent element, namely BODY. The H1 element is also a child
element of BODY but the second rule overrides the inherited value. In
CSS there are often such conflicts between different values, and this
specification describes how to resolve them.
<P>CSS 2.1 has more than 90 properties, including <span
class="propinst-color">'color'</span>. Let's look at some of the
others:
<PRE class="example">
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN">
<HTML>
<HEAD>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
body {
font-family: "Gill Sans", sans-serif;
font-size: 12pt;
margin: 3em;
}
</STYLE>
</HEAD>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
</PRE>
<P>The first thing to notice is that several declarations are grouped
within a block enclosed by curly braces ({...}), and separated by
semicolons, though the last declaration may also be followed by a
semicolon.
<P>The first declaration on the BODY element sets the font family to
"Gill Sans". If that font is not available, the user agent (often
referred to as a "browser") will use the 'sans-serif' font family
which is one of five generic font families which all users agents
know. Child elements of BODY will inherit the value of the <span
class="propinst-font-family">'font-family'</span> property.
<P>The second declaration sets the font size of the BODY element to
12 points. The "point" unit is commonly used in print-based typography
to indicate font sizes and other length values. It's an example of an
absolute unit which does not scale relative to the environment.
<P>The third declaration uses a relative unit which scales with regard
to its surroundings. The "em" unit refers to the font size of the
element. In this case the result is that the margins around the BODY
element are three times wider than the font size.
<H2><a name="xml-tutorial">A brief CSS 2.1 tutorial for XML</a></H2>
<p>This section is non-normative.
<P>CSS can be used with any structured document format, for example
with applications of the eXtensible Markup Language [[-XML10]]. In
fact, XML depends more on style sheets than HTML, since authors can
make up their own elements that user agents do not know how to
display.
<P>Here is a simple XML fragment:
<PRE class="xml-example">
<ARTICLE>
<HEADLINE>Fredrick the Great meets Bach</HEADLINE>
<AUTHOR>Johann Nikolaus Forkel</AUTHOR>
<PARA>
One evening, just as he was getting his
<INSTRUMENT>flute</INSTRUMENT> ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
</PARA>
</ARTICLE>
</PRE>
<P>To display this fragment in a document-like fashion, we must first
declare which elements are inline-level (i.e., do not cause line breaks) and
which are block-level (i.e., cause line breaks).
<PRE class="example">
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
</PRE>
<P>The first rule declares INSTRUMENT to be inline and the second
rule, with its comma-separated list of selectors, declares all the
other elements to be block-level. Element names in XML are
case-sensitive, so a selector written in lowercase (e.g., 'instrument')
is different from uppercase (e.g., 'INSTRUMENT').
<P>One way of linking a style sheet to an XML document is to use
a processing instruction:
<PRE class="xml-example">
<?xml-stylesheet type="text/css" href="bach.css"?>
<ARTICLE>
<HEADLINE>Fredrick the Great meets Bach</HEADLINE>
<AUTHOR>Johann Nikolaus Forkel</AUTHOR>
<PARA>
One evening, just as he was getting his
<INSTRUMENT>flute</INSTRUMENT> ready and his
musicians were assembled, an officer brought him a list of
the strangers who had arrived.
</PARA>
</ARTICLE>
</PRE>
<P>A visual user agent could format the above example as:
<div class="figure">
<p><img src="images/bach1.png" alt="Example rendering">
</div>
<P>Notice that the word "flute" remains within the paragraph since it
is the content of the inline element INSTRUMENT.
<P>Still, the text is not formatted the way you would expect. For
example, the headline font size should be larger than then the rest of
the text, and you may want to display the author's name in italic:</P>
<PRE class="example">
INSTRUMENT { display: inline }
ARTICLE, HEADLINE, AUTHOR, PARA { display: block }
HEADLINE { font-size: 1.3em }
AUTHOR { font-style: italic }
ARTICLE, HEADLINE, AUTHOR, PARA { margin: 0.5em }
</PRE>
<P>A visual user agent could format the above example as:
<div class="figure">
<p><img src="images/bach2.png" alt="Example rendering">
</div>
<P>Adding more rules to the style sheet will allow you to further
describe the presentation of the document.
<H2><a name="processing-model">The CSS 2.1 processing model</a></H2>
<p>This section up to but not including its subsections is
non-normative.
<P>This section presents one possible model of how user
agents that support CSS work. This is only a conceptual model; real
implementations may vary.
<P>In this model, a user agent processes a source
by going through the following steps:</P>
<ol>
<li>Parse the source document and create a <a
href="conform.html#doctree">document tree</a>.</li>
<li>Identify the target <a href="media.html">media type</a>.
<li>Retrieve all style sheets associated with the document that are
specified for the target <a href="media.html">media type</a>.
<li>Annotate every element of the document tree by assigning a single
value to every <a href="syndata.html#properties">property</a> that is
applicable to the target <a href="media.html">media type</a>.
Properties are assigned values according to the mechanisms described
in the section on <a href="cascade.html">cascading and
inheritance</a>.
<P>Part of the calculation of values depends on the formatting
algorithm appropriate for the target <a href="media.html">media
type</a>. For example, if the target medium is the screen, user agents
apply the <a href="visuren.html">visual formatting model</a>.
<li>From the annotated document tree, generate a
<span class="index-def" title="formatting structure">
<a name="formatting-structure"><dfn>formatting
structure</dfn></a></span>. Often, the formatting structure closely
resembles the document tree, but it may also differ significantly,
notably when authors make use of pseudo-elements and generated content.
First, the formatting structure need not be "tree-shaped" at all -- the
nature of the structure depends on the implementation. Second, the
formatting structure may contain more or less information than the
document tree. For instance, if an element in the document tree has a
value of 'none' for the <span
class="propinst-display">'display'</span> property, that element will
generate nothing in the formatting structure. A list element, on the
other hand, may generate more information in the formatting structure:
the list element's content and list style information (e.g., a bullet
image).
<P>Note that the CSS user agent does not alter the document tree
during this phase. In particular, content generated due to style
sheets is not fed back to the document language processor (e.g., for
reparsing).
<li>Transfer the formatting structure to the target medium (e.g., print
the results, display them on the screen, render them as speech,
etc.).
</ol>
<h3><a name="the-canvas">The canvas</a></h3>
<P>For all media, the term <span class="index-def" title="canvas"> <a
name="canvas"><dfn>canvas</dfn></a></span> describes "the space where
the formatting structure is rendered." The canvas is infinite for each
dimension of the space, but rendering generally occurs within
a finite region of the canvas, established by the user agent
according to the target medium. For instance, user agents rendering
to a screen generally impose a minimum width and choose an initial
width based on the dimensions of the <a href="visuren.html#viewport">
viewport</a>. User agents rendering to a page generally impose
width and height constraints. Aural user agents may impose limits
in audio space, but not in time.
<h3><a name="addressing">CSS 2.1 addressing model</a></h3>
<P> CSS 2.1 <a href="selector.html">selectors</a> and properties allow
style sheets to refer to the following parts of a document
or user agent:</p>
<ul>
<li>Elements in the document tree and certain relationships between
them (see the section on <a href="selector.html">selectors</a>).
<li>Attributes of elements in the document tree, and values of those
attributes (see the section on <a
href="selector.html#attribute-selectors">attribute selectors</a>).
<li>Some parts of element content (see the <a
href="selector.html#first-line">:first-line</a> and <a
href="selector.html#first-letter">:first-letter</a> pseudo-elements).
<li>Elements of the document tree when they are in a certain state
(see the section on <a href="selector.html#pseudo-classes">pseudo-classes</a>).
<li>Some aspects of the <a href="#canvas">canvas</a> where
the document will be rendered.
<li>Some system information (see the section on <a href="ui.html">user
interface</a>).
</ul>
<H2><a name="design-principles">CSS design principles</a></H2>
<p>This section is non-normative.
<P>CSS 2.1, as CSS2 and CSS1 before it, is based on a set of design principles:</p>
<UL>
<LI>
<P><STRONG>Forward and backward compatibility</STRONG>. CSS 2.1 user
agents will be able to understand CSS1 style sheets. CSS1 user agents
will be able to read CSS 2.1 style sheets and discard parts they do not
understand. Also, user agents with no CSS support will be able to
display style-enhanced documents. Of course, the stylistic
enhancements made possible by CSS will not be rendered, but all
content will be presented.</P>
</LI>
<LI>
<P><STRONG>Complementary to structured documents</STRONG>. Style
sheets complement structured documents (e.g., HTML and XML
applications), providing
stylistic information for the marked-up text. It should be easy to
change the style sheet with little or no impact on the markup.</P>
</LI>
<LI>
<P><STRONG>Vendor, platform, and device independence</STRONG>. Style
sheets enable documents to remain vendor, platform, and device
independent. Style sheets themselves are also vendor and platform
independent, but CSS 2.1 allows you to target a style sheet for a group of
devices (e.g., printers).</P>
</LI>
<LI>
<P><STRONG>Maintainability</STRONG>. By pointing to style sheets from
documents, webmasters can simplify site maintenance and retain
consistent look and feel throughout the site. For example, if
the organization's background color changes, only one file needs to be
changed.</P>
</LI>
<LI>
<P><STRONG>Simplicity</STRONG>. CSS is a simple style language which
is human readable and writable. The CSS properties are kept
independent of each other to the largest extent possible and there is
generally only one way to achieve a certain effect.</P>
</LI>
<LI>
<P><STRONG>Network performance</STRONG>. CSS provides for compact
encodings of how to present content. Compared to images or audio
files, which are often used by authors to achieve certain rendering
effects, style sheets most often decrease the content size. Also,
fewer network connections have to be opened which further increases
network performance.</P> </LI>
<LI>
<P><STRONG>Flexibility</STRONG>. CSS can be applied to content in
several ways. The key feature is the ability to cascade style
information specified in the default (user agent) style sheet, user
style sheets, linked style sheets, the document head, and in
attributes for the elements forming the document body.</P>
</LI>
<LI>
<P><STRONG>Richness</STRONG>. Providing authors with a rich set of
rendering effects increases the richness of the Web as a medium of
expression. Designers have been longing for functionality commonly
found in desktop publishing and slide-show applications. Some of
the requested rendering effects conflict with device independence, but
CSS 2.1 goes a long way toward granting designers their requests.</P>
</LI>
<LI>
<P><STRONG>Alternative language bindings</STRONG>. The set of CSS
properties described in this specification form a consistent
formatting model for visual and aural presentations. This formatting
model can be accessed through the CSS language, but bindings to other
languages are also possible. For example, a JavaScript program may
dynamically change the value of a certain element's <span
class="propinst-color">'color'</span> property.</P>
</LI>
<LI>
<P><STRONG>Accessibility</STRONG>. Several CSS
features will make the Web more accessible
to users with disabilities:</p>
<UL>
<LI>Properties to control font appearance allow authors
to eliminate inaccessible bit-mapped text images.
<li>Positioning properties allow authors to eliminate
mark-up tricks (e.g., invisible images) to force layout.
<LI>The semantics of
<tt>!important</tt> rules mean that users with
particular presentation requirements
can override the author's style sheets.
<li>The 'inherit' value for all properties
improves cascading generality and allows for
easier and more consistent style tuning.
<li>Improved media support, including media groups and the
braille, embossed, and tty media types, will allow users and
authors to tailor pages to those devices.
</UL>
<div class="note"><P> <em><strong>Note.</strong> For more information
about designing accessible documents using CSS and HTML, see [[-WAI-PAGEAUTH]].</em>
</div>
</UL>
</BODY>
</HTML>
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