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[css2] Added processing model (new draft)
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‎css2/intro.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: intro.src,v 1.11 1997-11-21 00:06:36 ian Exp $ -->
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<!-- $Id: intro.src,v 1.12 1997-11-26 00:06:32 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>Introduction to CSS2</TITLE>
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<ul>
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<li> the type of link: to a "stylesheet".
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<li> the location of the stylesheet via a URL. Hhere, the file
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<li> the location of the stylesheet via a URL. Here, the file
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<samp>bach.css</samp> in the same directory as the source HTML document.
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<li>the type of style sheet being linked: "text/css".
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</ul>
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</PRE>
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<!-- Add image, printing, info, positioning, fonts -->
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<H2>The CSS2 processing model</H2>
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<P>The following section presents one possible model of how user
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agents that support CSS work. This is only a conceptual model; real
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implementations may vary.
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<P>In this model, a user agent processes a source document
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from parsing to rendering as follows:
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<ol>
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<li>Parse the source document and create a <a
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href="convent.html#doctree">document tree</a> from the source
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document.</li>
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<li>Identify the target <a href="media.html">media type</a>.
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<li>Retrieve all style sheets associated with the document that are
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specified for the target <a href="media.html">media type</a>.
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<li>Annotate every node of the document tree by assigning a single
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value to every <a href="syndata.html#properties">property</a> that is
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applicable to the target <a href="media.html">media type</a>.
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Properties are assigned values according to the mechanisms described
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in the section on <a href="cascade.html">cascading and
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inheritance</a>.
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<P>Part of the calculation of values depends on the formatting
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algorithm appropriate for the target <a href="media.html">media
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type</a>. For example, if the target medium is the screen, user agents
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apply the <a href="flowobj.html">visual flow model</a>. If the
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destination medium is the printed page, user agents apply the <a
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href="page.html#page-model">page model</a>. If the destination medium
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is an aural rendering device (e.g., speech synthesizer), user agents
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apply the <a href="aural.html">aural rendering model</a>.
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<li>From the annotated document tree, generate a rendering structure.
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The rendering structure may differ significantly from the document
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tree. First , the rendering structure need not be "tree-shaped" at all
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-- the nature of the structure depends on the implementation.
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<P>Second the rendering structure may contain more or less information
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than the document tree. For instance, if an element in the document
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tree has a value of 'none' for the <span
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class="propinst-display">'display'</span> property, that element will
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generate nothing in the rendering structure. A list element, on the
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other hand, may generate more information in the rendering structure:
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the list element's content and list style information (e.g., a bullet
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image).
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<!-- Is the next statement still true? If so, it should probably
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be made specific and moved to flowobj2.src -->
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<!--
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<P>If an element A is an ancestor of an element D, all rendering
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objects generated for A must be above the rendering objects
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rendered for D in the tree of rendering objects. For box
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and page box rendering objects, this implies that, to
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find a containing box for a node, a user agent need only search
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upward in the tree of rendering objects (not left, right, or down).
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-->
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<li>Transfer the rendering structure to the target medium (e.g., print
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the results, display them on the screen, render text as speech,
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etc.).
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</ol>
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<P>Step 1 lies outside the scope of this specification (see, for
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example, <a rel="biblioentry" href="./refs.html#ref-DOM"
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class="informref">[DOM]</a>).
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<P>Steps 2-5 are addressed by the bulk of this specification.
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<P>Step 6 lies outside the scope of this specification (and may not
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exist in a given implementation).
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<P>The majority of transfer issues in step 7 lie outside the scope of
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this specification. However, CSS2 addresses these issues:
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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., <a href="fonts.html">fonts</a>)?
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<li>How do style sheets interact
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with system resources (e.g., <a href="ui.html">cursors and colors</a>)?
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<li>How do style sheet properties assist certain devices
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(e.g., <a href="page.html">page orientation for a printer</a>)?
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</ul>
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<H2>Design principles behind CSS2</H2>
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<P><EM>This section is under construction</EM>
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<UL>
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<LI>backward compatibility
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<LI>complementary to structured documents
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<LI>network performance
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</UL>
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<P><EM>This section will be expanded</EM>
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</BODY>
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</HTML>

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