[css-values-5] Define inherit() for custom properties #2864

Author: fantasai

css-values-5/Overview.bs

@@ -1476,6 +1476,37 @@ Custom Property References: the ''var()'' notation</h3>
 	see the <a href="https://www.w3.org/TR/css-variables/#using-variables">CSS Custom Properties for Cascading Variables Module</a>.
 	[[CSS-VARIABLES]]
 
+<h3 id="inherit-notation">
+Inherited Value References: the ''inherit()'' notation</h3>
+
+	Like the ''inherit'' keyword, the <dfn>inherit()</dfn> [=functional notation=]
+	resolves to the [=computed value=] of a property on the parent.
+	Rather than resolving to the value of the same property, however,
+	it resolves to the tokenized [=computed value=]
+	of the property specified as its first argument.
+	Its second argument, if present, is used as a fallback
+	in case the first argument resolves to the [=guaranteed-invalid value=].
+
+	''inherit()'' is an [=arbitrary substitution function=]
+	whose syntax is defined as:
+
+	<pre class=prod>
+		<dfn><<inherit()>></dfn> = inherit( <<custom-property-name>>, <<declaration-value>>? )
+	</pre>
+
+	To <dfn export>[=resolve an arbitrary substitution function|resolve an inherit() function=]</dfn>,
+	return the [=inherited value=] of the [=custom property=] specified by the first argument,
+	and (if specified) the fallback specified by the second argument.
+
+	Note: Future levels of CSS may allow specifying standard CSS properties in ''inherit()'';
+	however because the tokenization of [=computed values=]
+	is not fully standardized for all CSS properties,
+	this feature is deferred from Level 5.
+	Note that the [=computed value=] differs from the [=used value=],
+	and is not always the resolved value returned by {{getComputedStyle()}};
+	thus even if ''inherit(width)'' were allowed,
+	it would frequently return the keyword ''width/auto'', not the used <<length>>.
+
 <!-- Big Text: attr()
 
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