[css-contain-3] Support boolean context style() queries w3c#8127 (w3c#8729)

Per resolution in w3c#8127, style() queries can query properties without a
value which evaluates to true if the computed value is different from
the initial value for that property.

Co-authored-by: Rune Lillesveen <futhark@chromium.org>

Author: lilles

css-contain-3/Overview.bs

@@ -602,6 +602,7 @@ Container Queries: the ''@container'' rule</h3>
 	<dfn><<style-in-parens>></dfn>     = ( <<style-query>> )
 	                      | ( <<style-feature>> )
 	                      | <<general-enclosed>>
+	<dfn><<style-feature>></dfn>			 = <<ident>> : <<declaration-value>> | <<ident>>
 	</pre>
 
 	The keywords ''container-name/none'', ''and'', ''not'', and ''or''
@@ -910,6 +911,11 @@ Style Container Features</h3>
 	(which is also [=computed value|computed=] with respect to the [=query container=]),
 	unknown if the property or its value is invalid or unsupported,
 	and false otherwise.
+
+	A [=style feature=] can also be a property name without a value. Such
+	features evaluate to true if the [=computed value=] is different from the [=initial=]
+	value for the given [=property=].
+
 	The boolean syntax and logic combining [=style features=] into a [=container style query|style query=]
 	is the same as for [=CSS feature queries=].
 	(See ''@supports''. [[!CSS-CONDITIONAL-3]])