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[css2] Edits based on Hakons extensive comments + Chris/Bert replies
--HG-- extra : convert_revision : svn%3A73dc7c4b-06e6-40f3-b4f7-9ed1dbc14bfc/trunk%40977
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‎css2/cascade.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: cascade.src,v 2.1 1998-02-18 11:45:31 howcome Exp $ -->
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<!-- $Id: cascade.src,v 2.2 1998-02-18 23:37:40 ijacobs 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>Assigning property values, Cascading, and Inheritance</TITLE>
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calculation: the value is determined through specification (the
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"specified value"), then resolved into an absolute value if necessary
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(the "computed value"), and finally transformed according to
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the limitations of the local environment (the "actual value") if
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necessary.
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the limitations of the local environment (the "actual value").
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<h3><a name="specified-value">Specified values</a></h3>
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<h3>
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<span class="index-def" title="specified value">
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<a name="specified-value">Specified values</a></span></h3>
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<P>User agents must first assign a <span class="index-def"
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title="specified value">specified value</span> to a property
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based on the following mechanisms (in order of precedence):</p>
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<P>User agents must first assign a specified value to a property based
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on the following mechanisms (in order of precedence):</p>
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<ol>
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<li>If the <a href="#cascade">cascade</a> results in a value,
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use it.
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use it. If the value is 'inherit', use the value of the parent
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element.
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<li>Otherwise, if the value is <a href="#inheritance">inherited</a>,
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use it.
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in the property's definition.
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</ol>
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<h3><a name="computed-value">Computed values</a></h3>
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<P>Specified values may be "absolute" (e.g., the color value 'red' or
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the constant value 'hidden') or "relative" (e.g., the variable value
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'auto', the font-related value '1.2em', pixel values, percentage values,
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etc.). Each value must be transformed into a <span class="index-def"
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title="computed value">computed value</span> according to
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algorithms described in this specification.
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<P>The root of the <a href="conform.html#doctree">document tree</a>
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cannot use values from the parent element since it has no parent. As a
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substitute, the initial value should be used whenever the root element
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needs a value from a parent element.
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<P>When the root of the document tree has a property whose specified
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value is inherited and has relative units, the computed value is the
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percentage times the property's initial value.
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<h3><span class="index-def" title="computed value">
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<a name="computed-value">Computed values</a>
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</span>
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</h3>
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<div class="example"><P>
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For example, with an HTML document and the following style sheet:
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<pre>
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HTML { font-size: 120% }
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</pre>
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<P>Specified values may be absolute (i.e., they are not specified
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relative to another value, as in 'red' or '2mm') or relative (i.e.,
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they are specified relative to another value, as in 'auto', '2em', and
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'12%'). For absolute values, no computation is needed to find the
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computed value. Relative values, on the other hand, must be
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transformed into computed <a href="syndata.html#values">values</a>
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as described in the chapter on <a href="syndata.html">syntax</a>.
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Each property defines the meaning of that property's percentage
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values.
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<p>the computed value for <span
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class="prop-inst-font-size">'font-size'</span> will be 120% of the
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initial value of the <span
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class="propinst-font-size">'font-size'</span> property. The initial
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value of <span class="propinst-font-size">'font-size'</span> is
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defined to be 'medium', so the actual value is 20% larger than
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'medium'. The actual value that this results in depends on
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the current environment.
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</div>
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<h3><span class="index-def" title="actual value">
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<a name="actual-value">Actual values</a></span></h3>
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<h3><a name="actual-value">Actual values</a></h3>
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<P>A computed value is in principle ready to be used, but a user agent
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may not be able to make use of the value in a given environment. For
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example, a user agent may not have a specified font size available, in
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which case the user agent must approximate the computed value. The
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actual value is the computed value after any approximations have been
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applied.
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<P>A computed value has an absolute meaning but a user agent may not
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be able to respect this meaning in a given environment. For example, a
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user agent may not have a specified font size available, in which case
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the user agent must approximate the computed value. Computed values
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that are transformed to match the current environment are called <span
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class="index-def" title="actual value">actual values</span>.
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<P>Only actual values are inherited.
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<P>Whenever an element inherits a value, it inherits an
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actual value.
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<H2><a name="inheritance">Inheritance</a></h2>
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<P>Some <a href="#actual-value">actual values</a> are <span
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class="index-def" title="inheritance of property
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values">inherited</span> by the descendants of a node in the <a
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href="conform.html#doctree">document tree</a>. Each property
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definition specifies whether its value may be inherited.
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<P> Some propeties are inherited, i.e., a descendant of a node in the
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<a href="conform.html#doctree">document tree</a> will use the node's
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actual value for a property unless the property is specified for that
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descendant.
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<P>All properties accept the 'inherit' keyword. For an inherited
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property, this value is merely an explicit statement of the
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inheritance. For a property that is not inherited according to its
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definition, this value forces inheritance.
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<div class="html-example"><P>
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Suppose there is an H1 element with an emphasized element inside:
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Suppose there is an H1 element with an emphasizing element (EM)
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inside:</p>
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<PRE>
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&lt;H1&gt;The headline &lt;EM&gt;is&lt;/EM&gt; important!&lt;/H1&gt;
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the HTML parser will infer the missing tag.
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<div class="example"><P>
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For example, since the <span class="propinst-color">'color'</span>
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property is inherited, all descendants of the BODY element will
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inherit the color 'black':
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For example, these rules cause the <span
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class="propinst-color">'color'</span> property on the BODY element to
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be inherited by all descendants of the BODY element:
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<PRE>
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BODY { color: black; }
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</PRE>
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<P>Not all style properties are inherited, for example <span
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class="propinst-background">'background'</span>. But in the case of
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<span class="propinst-background">'background'</span>, the <em>visual
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effect</em> may be propagated to descendants. A
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background specified for the root element, as in:</p>
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<PRE>
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BODY {
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color: black;
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background: url(texture.gif) white;
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}
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BODY { background: url(texture.gif) white; }
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</PRE>
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<P> In this example, all descendants of the BODY element inherit the
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<span class="propinst-color">'color'</span> property.
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<P>will shine through any descendants unless they specify a value for
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<span class="propinst-background-color">'background-color'</span>
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other than 'transparent' (the initial value).
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</div>
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<P>Not all style properties are inherited. For example, the <span
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class="propinst-background">'background'</span> property is not
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inherited. (However, due to the initial value of 'transparent' on the
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<span class="propinst-background">'background'</span> property, the
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parent's background shines through.)
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<div class="example"><P>
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The following example illustrates that <a href="#specified-value">
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specified</a> percentage values are not inherited; only
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document or linked externally.
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<li><strong>User</strong>: The user may be able to specify style
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information for a document. The user agent may provide an interface
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that "generates" a user style sheet (or must behave as if it did).
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information for a particular document. For example, the user may
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specify a file that contains a style sheet or the user agent may
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provide an interface that generates a user style sheet (or behave as
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if it did).
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<li><strong>User agent</strong>: <a
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href="conform.html#conformance">Conforming user agents</a> must apply

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