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[css2] will contain generic process model
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‎css2/process.src‎

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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<html lang="en">
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<!-- $Id: process.src,v 1.1 1997-10-29 04:28:19 ian Exp $ -->
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<HEAD>
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<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
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<TITLE>CSS2 processing model</TITLE>
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<LINK rel="next" href="syndata.html">
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<LINK rel="previous" href="convent.html">
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<LINK rel="STYLESHEET" href="style/default.css" type="text/css">
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</HEAD>
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<BODY>
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<H1 align="center">CSS2 processing model</H1>
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<H2>Introduction to the CSS2 processing model</H2>
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In this section we present a model that describes how
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user agents may implement style sheets. The model consists
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of the following steps:
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<ol>
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<li>Create a <a href="#doctree">document tree</a>.
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<li>From the document tree, generate a tree of <a
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href="#rendobj">rendering objects</a> based on the
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destination medium, style rules, and formatting algorithms.
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<li>Transfer the tree of <a href="#rendobj">rendering objects</a> to
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the destination <a href="media.html">medium</a> (e.g., print the
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results, display them on the screen, render text as speech, etc.).
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</ol>
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<P>Step one lies outside the scope of this specification (see, for
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example, <a rel="biblioentry" href="./refs.src#ref-DOM">[DOM]</a>).
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<P>Step two is addressed by the bulk of this specification:
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<ul>
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<li>For what <a href="media.html">media</a> can style sheets present
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documents?
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<li>How do competing style rules interact?
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<li>What formatting algorithms apply for a given medium?
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</ul>
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<P>Step three is addressed in part by this specification:
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<ul>
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<li>What can user agents do when certain system resources are
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not available (e.g., fonts)?
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<li>How do style sheets interact
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with system resources (cursors, colors, fonts)?
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<li>How do style sheet properties assist certain devices
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(e.g., page orientation for a printer)?
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</ul>
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<P>Please note that most transfer issues lie outside the scope of this
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specification.
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<H2><a name="doctree">The document tree</a></H2>
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<P>User agents transform a document written in the document language
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into a <dfn>document tree</dfn> where every element except one has
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exactly one <span class="index-def" title="parent">parent</span>
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element. (See the SGML (<a href="refs.html#ref-ISO8879">[ISO8879]</a>)
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and XML (<a href="refs.html#ref-XML">[XML]</a>) specifications for the
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definition of parent.) The one exception is the <span
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class="index-def" title="root">root</span> element, which has no
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parent. An element A is called a <span class="index-def"
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title="child">child</span> of an element B, if and only if B is the
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parent of A.
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<P>An element A is called a <span class="index-def"
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title="ancestor">ancestor</span> of an element B, if either (1) A is
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the parent B, or (2) A is the parent of some element C that is an
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ancestor of B.
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<P>An element A is called a <span class="index-def"
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title="descendant">descendant</span> of an element B, if and only if B
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is an ancestor of A.
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<div class="example"><P>
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For example, the following HTML document:
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<PRE>
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&lt;HTML&gt;
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&lt;TITLE&gt;My home page&lt;/TITLE&gt;
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&lt;BODY&gt;
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&lt;H1&gt;My home page&lt;/H1&gt;
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&lt;P&gt;Welcome to my home page! Let me tell you about my favorite
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composers:
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&lt;UL&gt;
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&lt;LI&gt; Elvis Costello
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&lt;LI&gt; Johannes Brahms
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&lt;LI&gt; Georges Brassens
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&lt;/UL&gt;
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&lt;/BODY&gt;
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&lt;/HTML&gt;
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</PRE>
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<P>results in the following tree:</P>
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<IMG src="images/doctree.gif" alt="Sample document tree">
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<P>Notice that although the source program does not explicitly refer
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to the HEAD element, it is inferred during parsing and becomes part of
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the document tree.
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</div>
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<H2><a name="renderobj">Rendering objects</a></H2>
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<P>Once a user agent has a document tree, it generates a tree of
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rendering objects. A <dfn>rendering object</dfn> is defined by a set
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of CSS properties and a formatting algorithm. The properties and
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algorithsm generally relate to the intended target medium. Thus, a
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rendering object destined to be printed requires certain CSS
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properties (e.g., related to margins, widths, heights, colors,
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positioning, etc.) while a rendering object destined for a speech
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synthesizer requires others (e.g., volume, pitch, voice family, etc.).
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<ul>
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<li><em>The destination <a href="media.html">medium</a>.</em> For instance,
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if the document is to be read aloud, the rendering tree will be much
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different than if the document is to be laid out on paper.
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<li><em>Style rules</em>. CSS2 style rules
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</ul>
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<!--
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<ol>
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<li>Style sheets are <a href="cascade.html">cascaded</a> and <a
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href="#inheritance">inheritance</a> rules applied so that for each node in
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the document tree, each property has a single value.
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property.
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<li>The user agent generates zero or more <dfn>rendering objects</dfn>
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for each node in the document tree and creates a tree of these
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rendering objects. The type of the rendering object depends on the
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destination <a href="media.html">media type</a>. For screens, for
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example, a rendering object would be a box placed according to the
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<a href="flowobj.html">visual rendering model</a>. For paper output, a
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rendering object would be a box placed according to the <a
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href="page.html">page model</a>. For speech synthesizers, a
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rendering object would be speakable data with associated aural
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properties (see the section on <a href="aural.html">aural style
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sheets</a>).
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<li>These rendering objects are transferred to the destination medium.
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For example, abstract pages are printed to sheets of paper,
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audio rendering objects are transferred to a speech synthesizer, etc.
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The details of any transfer lie outside the scope of this
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specification.
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</ol>
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-->
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</BODY>
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</html>
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<!-- Keep this comment at the end of the file
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Local variables:
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mode: sgml
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sgml-declaration:"~/SGML/HTML4.decl"
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sgml-default-doctype-name:"html"
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sgml-minimize-attributes:t
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sgml-nofill-elements:("pre" "style" "br")
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sgml-live-element-indicator:t
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End:
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-->
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