[css-conditional-3] Add .matches to @media and @supports, w3c#4240

Author: svgeesus

css-conditional-3/Overview.bs

@@ -793,6 +793,7 @@ The {{CSSMediaRule}} interface represents a ''@media'' at-rule:
 [Exposed=Window]
 interface CSSMediaRule : CSSConditionRule {
     [SameObject, PutForwards=mediaText] readonly attribute MediaList media;
+    readonly attribute bool matches;
 };
 </pre>
 
@@ -808,6 +809,13 @@ js/CSSConditionRule.html
     for the list of media queries specified with the ''@media'' at-rule.
     <wpt title=".media returns a MediaList matching the @media condition."></wpt>
 
+  <dt><code>matches</code> of type {{bool}}, readonly
+    <dd>The <code>matches</code> attribute returns true
+      if the rule is in an stylesheet attached to a document
+      whose {{Window}} matches this rule’s {{CSSMediaRule/media}} [=media query=],
+      and returns false otherwise.
+      <wpt title=".matches matches the media query, returns boolean."></wpt>
+
   <dt><code>conditionText</code> of type <code>CSSOMString</code> (CSSMediaRule-specific definition for attribute on CSSConditionRule)
   <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
     on getting, must return the value of <code>media.mediaText</code> on the rule.
@@ -823,6 +831,7 @@ The {{CSSSupportsRule}} interface represents a ''@supports'' rule.
 <pre class='idl'>
 [Exposed=Window]
 interface CSSSupportsRule : CSSConditionRule {
+  readonly attribute bool matches;
 };
 </pre>
 
@@ -832,6 +841,11 @@ js/CSSConditionRule.html
 <wpt title="CSSSupportsRule represents an @supports rule."></wpt> -->
 
 <dl class='idl-attributes'>
+  <dt><code>matches</code> of type {{bool}}, readonly
+    <dd>The <code>matches</code> attribute returns the evaluation of
+      the [=CSS feature query=] represented in {{CSSConditionRule/conditionText}}.
+      <wpt title="CSSSupportsRule.matches returns true if matches feature query"></wpt>
+
   <dt><code>conditionText</code> of type <code>CSSOMString</code> (CSSSupportsRule-specific definition for attribute on CSSConditionRule)
   <dd>The <code>conditionText</code> attribute (defined on the <code>CSSConditionRule</code> parent rule),
     on getting, must return the condition that was specified,