conform.html

CSS lets learn this step by step...

<dt><strong><a name="x29"><span class="index-def"
title="sibling">Sibling</span></a></strong>

<dd>An element A is called a sibling of an element B, if and only if B
and A share the same parent element. Element A is a preceding sibling
if it comes before B in the document tree. Element B is a following
sibling if it comes after B in the document tree.

<dt><strong><span class="index-def" title="preceding
element|element::preceding"><a name="preceding">Preceding element</a></span></strong>

<dd>An element A is called a preceding element of an element B, if and
only if (1) A is an ancestor of B or (2) A is a preceding sibling of
B.

<dt><strong><a name="x32"><span class="index-def" title="following
element|element::following">Following element</span></a></strong>

<dd>An element A is called a following element of an element B, if and
only if B is a preceding element of A.

<dt><strong><strong><a name="author">Author</a></strong></strong>
<dd>An author is a person who writes
documents and associated style sheets. An 
<a name="x34"><span class="index-def" title="authoring tool"><strong>authoring tool</strong></span></a>
generates documents and associated style sheets.</dd>

<dt><strong><strong><a name="user">User</a></strong></strong> 
<dd> A user is a person who interacts with a user agent to view, hear, or
otherwise use a document and its associated style sheet. The user
may provide a personal style sheet that encodes personal
preferences.
</dd>

<dt><strong><strong><a name="user-agent">User agent (UA)</a></strong></strong>
<dd>A <a name="x35"><span class="index-def" title="user agent|UA">user
agent</span></a> is any program that interprets a document written in
the document language and applies associated style sheets according
to the terms of this specification. A user agent may display a
document, read it aloud, cause it to be printed, convert it
to another format, etc.</dd>
</dl>

<div class="html-example">
<p>Here is an example of a source document encoded in HTML:

<PRE>
&lt;!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0//EN"&gt;
&lt;HTML&gt;   
  &lt;TITLE&gt;My home page&lt;/TITLE&gt;
  &lt;BODY&gt;
    &lt;H1&gt;My home page&lt;/H1&gt;
    &lt;P&gt;Welcome to my home page! Let me tell you about my favorite
		composers:
    &lt;UL&gt;
      &lt;LI&gt; Elvis Costello
      &lt;LI&gt; Johannes Brahms
      &lt;LI&gt; Georges Brassens
    &lt;/UL&gt;
  &lt;/BODY&gt;
&lt;/HTML&gt;
</PRE>

<P>This results in the following tree:</P>

<div class="figure">
<P><IMG src="images/doctree.gif" alt="Sample document tree"><SPAN class="dlink">&nbsp;&nbsp;&nbsp;<A name="img-doctree" href="images/longdesc/doctree-desc.html" title="Long description for sample document tree diagram">[D]</A></SPAN></p>
</div>

<P>According to the definition of HTML, HEAD elements will be inferred
during parsing and become part of the document tree even if the HEAD
tags are not in the document source. Similarly, the parser knows where
the P and LIs end, even though there are no &lt;/P&gt; and &lt;/LI&gt; tags
in the source.
</div>


<h2>3.2 <a name="conformance">Conformance</a></h2>

<P>This section defines <a name="x37"><span class="index-def"
title="conformance">conformance</span></a> with the CSS2
specification only. There may be other levels of CSS in the future
that may require a user agent to implement a different set of features
in order to conform.

<P>In general, the following points must be observed by a user agent
claiming conformance to this specification:</P>

<ol>
<li>It must support one or more of the
CSS2 <a href="media.html">media types</a>.
<li>For each source document, it must attempt to retrieve all 
associated style sheets that are appropriate for the supported media
types. If it cannot retrieve all associated style sheets (for instance,
because of network errors), it must display the document using those
it can retrieve.
<li>It must parse the style sheets according to this specification.
In particular, it must recognize all at-rules, blocks, declarations,
and selectors (see the <a href="grammar.html">grammar of CSS2</a>).
If a user agent encounters a property that applies for a supported
media type, the user agent must parse the value according to the property
definition. This means that the user agent must accept all valid
values and must  
<a name="x38"><span class="index-inst" title="ignore">ignore</span></a> declarations with
invalid values. User
agents must <a name="x39"><span class="index-inst" title="ignore">ignore</span></a>
rules that apply to unsupported <a href="media.html">media
types</a>.

<li>For each element in a <a href="#doctree">document tree</a>, it must
assign a value for every applicable property according to
the property's definition and the rules of 
<a href="cascade.html">cascading and inheritance</a>.

<li>If the source document comes with alternate style sheets (such as
with the "alternate" keyword in HTML 4.0 <a href="refs.html#ref-HTML40" rel="biblioentry" class="noxref"><span class="informref">[HTML40]</span></a>), the UA must
allow the user to select one from among these style sheets and apply
the selected one.
</ol>

<P>Not every user agent must observe every point, however:</p>

<ul>
<li>A user agent that <em>inputs</em> style sheets must
respect points 1 - 3.

<li>An authoring tool is only required to output <a
href="#valid-style-sheet">valid style sheets</a>

<li>A user agent that <em>renders</em> a document with associated style
sheets must respect points 1 - 5 and render the document
according to the media-specific requirements set forth in this
specification. <a href="cascade.html#actual-value">Values</a> 
may be approximated when required by the user agent.
</ul>

<P>The inability of a user agent to implement part of this
specification due to the limitations of a particular device (e.g., a
user agent cannot render colors on a monochrome monitor or page) does
not imply non-conformance. </P>

<p>This specification recommends that a user agent allow the user to
specify user style sheets.



<h2><a name="q3">3.3 Error conditions</a></h2>

<P>In general, this document does not specify error handling behavior
for user agents (e.g., how they behave when they cannot find
a resource designated by a URI).

<P>However, user agents must observe the <a
href="syndata.html#parsing-errors">rules for handling parsing errors</a>.

<P>Since user agents may vary in how they handle error conditions,
authors and users must not rely on specific error recovery behavior.


<H2>3.4 <span class="index-def" title="text/css"><a
name="text-css">The text/css content type</a></span></H2>

<p>CSS style sheets that exist in separate files are sent over the
Internet as a sequence of bytes accompanied by encoding information
(see <a href="refs.html#ref-HTML40" rel="biblioentry" class="noxref"><span class="normref">[HTML40]</span></a>, chapter 5).  The structure of the transmission,
termed a <span class="index-def" title="message entity"><a
name="message-entity"><strong>message entity,</strong></a></span> is
defined by RFC 2045 and RFC 2068 (see <a href="refs.html#ref-RFC2045" rel="biblioentry" class="noxref"><span class="normref">[RFC2045]</span></a> and <a href="refs.html#ref-RFC2068" rel="biblioentry" class="noxref"><span class="normref">[RFC2068]</span></a>).  A
message entity with a content type of "text/css" represents an
independent CSS document. The "text/css" content type has
been registered by RFC 2138 (<a href="refs.html#ref-2318" rel="biblioentry" class="noxref"><span class="normref">[RFC2318]</span></a>).

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