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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html lang="en">
<!-- $Id: intro.src,v 2.22 1998-03-20 22:38:40 bbos Exp $ -->
<HEAD>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<TITLE>Introduction to CSS2</TITLE>
</HEAD>
<BODY>
<H1 align="center">Introduction to CSS2</H1>
<H2>A brief CSS2 tutorial for HTML</H2>
<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 [[-HTML40]]) and some basic desktop publishing terminology.
<P>We begin with a small HTML document:</p>
<PRE class="html-example">
<HTML>
<TITLE>Bach's home page</TITLE>
<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 blue, you can write the
following CSS rule:</P>
<PRE class="example">
H1 { color: blue }
</PRE>
<P>A CSS rule consists of two main parts: <a
href="selector.html">selector</a> ('H1') and declaration ('color:
blue'). The declaration has two parts: property ('color') and value
('blue'). 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) it will
determine the final presentation of the document.
<P> The HTML 4.0 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">
<HTML>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
H1 { color: blue }
</STYLE>
<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">
<HTML>
<TITLE>Bach's home page</TITLE>
<LINK rel="stylesheet" href="bach.css" type="text/css">
<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. <!--Here, the file
<samp>bach.css</samp> is in the same directory as the source HTML
document. -->
<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">
<HTML>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
BODY { color: red }
H1 { color: blue }
</STYLE>
<BODY>
<H1>Bach's home page</H1>
<P>Johann Sebastian Bach was a prolific composer.
</BODY>
</HTML>
</PRE>
<P>The style sheet now contains two rules: the first one sets the
color of the BODY element to 'red', while the second one sets the
color of the H1 element to 'blue'. Since no color value 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>CSS2 has more than 100 different properties and 'color' is one of
them. Let's look at some of the others:
<PRE>
<HTML>
<TITLE>Bach's home page</TITLE>
<STYLE type="text/css">
BODY {
font-family: "Gill Sans", sans-serif;
font-size: 12pt;
margin: 3em;
}
</STYLE>
<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 isn't 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 'font-family' 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.
<!-- Add image, printing, info, positioning, fonts -->
<!-- [commented out to spare newbies from the cruel world out there, -h&kon]
<div class="note"><P>
<p><em><strong>Note.</strong> To hide style sheets from older HTML user
agents, authors may comment out the content of the STYLE element.
Please consult the HTML 4.0 specification for more information.
</em>
</div>
-->
<H2>A brief CSS2 tutorial for XML</H2>
<P>CSS can be used with any structured document format, for example
the eXtensible Markup Language [[-XML10]]. In fact, XML depends more on
style sheets than HTML, since authors can make up their own elements
which user agents don't 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 <!--(though this rule
is not strictly necessary, because 'inline' is the default),[hmm,
perhaps initial value, but it's anyway too advanced, HWL]--> and the
second rule, with its comma-separated list of selectors, declares all
the other elements to be block-level.
<P>One proposal for 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:
<p><img src="images/bach1.gif" alt="Example rendering">
<P>Notice that the word "flute" remains within the paragraph since it
is the content of the inline element INSTRUMENT.
<P>Still, the text isn't formatted the way you would expect. For
example, the headline font size should be larger than then 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><img src="images/bach2.gif" alt="Example rendering">
<P>Adding more rules to the style sheet will allow you to further
improve the presentation of the document.
<H2><a name="processing-model">The CSS2 processing model</a></H2>
<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> from the source
document.</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 rendering model</a>. If the
destination medium is the printed page, user agents apply the <a
href="page.html#page-model">page model</a>. If the destination medium
is an aural rendering device (e.g., speech synthesizer), user agents
apply the <a href="aural.html">aural rendering model</a>.
<li>From the annotated document tree, generate a
<span class="index-def" title="rendering structure">
<a name="rendering-structure"><dfn>rendering
structure</dfn></a></span>. Often, the rendering 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 rendering structure need not be "tree-shaped" at all -- the
nature of the structure depends on the implementation. Second, the
rendering 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 rendering structure. A list element, on the
other hand, may generate more information in the rendering 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).
<!-- Is the next statement still true? If so, it should probably
be made specific and moved to visudet.src -->
<!--
<P>If an element A is an ancestor of an element D, all rendering
objects generated for A must be above the rendering objects
rendered for D in the tree of rendering objects. For box
and page box rendering objects, this implies that, to
find a containing box for an element, a user agent need only search
upward in the tree of rendering objects (not left, right, or down).
-->
<li>Transfer the rendering structure to the target medium (e.g., print
the results, display them on the screen, render them as speech,
etc.).
</ol>
<P>Step 1 lies outside the scope of this specification (see, for
example, [[-DOM]]).
<P>Steps 2-5 are addressed by the bulk of this specification.
<P>Step 6 lies outside the scope of this specification.</p>
<h3>The canvas</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 rendering 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>CSS2 addressing model</h3>
<P> CSS2 <a href="selector.html">selectors</a> and properties allow
style sheets to refer to the following characteristics of a document:</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>CSS design principles</H2>
<P>CSS2, as CSS1 before it, is based on a set of design principles:</p>
<UL>
<LI>
<P><STRONG>Forward and backward compatibility</STRONG>. CSS2 user
agents will be able to understand CSS1 style sheets. CSS1 user agents
will be able to read CSS2 style sheets and discard parts they don't
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), 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 CSS2 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>. CSS2 is more complex than CSS1, but
it remains 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 e.g., in desktop publishing and slide-show applications. Some of
the requested rendering effects conflict with device independence, but
CSS2 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>. Last, but not least, CSS will
increase accessibility to Web documents, not just for people with
handicaps, but for everybody, and even for robots. By retaining
textual information in text form, robots and human users alike will
have more options for digesting the content. Users can provide their
personal style sheets if author-suggested style sheets hinder
accessibility. The cascading mechanism combines and selects from
different style sheets.</P>
</LI>
</UL>
</BODY>
</HTML>
<!-- Keep this comment at the end of the file
Local variables:
mode: sgml
sgml-omittag:nil
sgml-shorttag:nil
sgml-minimize-attributes:nil
sgml-always-quote-attributes:t
sgml-indent-step:nil
sgml-indent-data:t
sgml-parent-document:nil
sgml-exposed-tags:nil
sgml-local-catalogs:nil
sgml-local-ecat-files:nil
End:
-->