[selectors-4] Add :is(), resolves #1170

Author: LeaVerou

selectors-4/Overview.bs

@@ -1075,6 +1075,48 @@ The Matches-any Pseudo-class: '':matches()''</h3>
 		<pre>*|*:matches(*:hover, *:focus) </pre>
 	</div>
 
+<h3 id="is">
+The Specificity-adjustment Pseudo-class: '':is()''</h3>
+
+The Specificity-adjustment pseudo-class '':is()'', is a functional pseudo-class
+with the same syntax and functionality as '':matches()''.
+Unlike '':matches()'', neither the pseudo-class, nor any of its arguments
+contribute to the specificity of the selector.
+
+This is useful for introducing filters in a selector while keeping it easy to override.
+
+<div class="example">
+	Below is a common example where the specificity heuristic fails
+	to match author expectations:
+
+	<pre>
+		a:not(:hover) {
+			text-decoration: none;
+		}
+
+		nav a {
+			/* Has no effect */
+			text-decoration: underline;
+		}
+	</pre>
+
+	However, by using '':is()'' the author can explicitly declare their intent:
+
+	<pre>
+		a:is(:not(:hover)) {
+			text-decoration: none;
+		}
+
+		nav a {
+			/* Works now! */
+			text-decoration: underline;
+		}
+	</pre>
+</div>
+
+Note: Future versions of this specification may introduce a second argument to explicitly set
+the specificity of each instance of this pseudo-class, which will default to zero.
+
 <h3 id="negation">
 The Negation Pseudo-class: '':not()''</h3>
 
@@ -3067,6 +3109,8 @@ Calculating a selector's specificity</h2>
 			The specificity of a '':matches()'' pseudo-class is replaced by
 			the specificity of its selector list argument.
 			(The full selector's specificity is equivalent to expanding out all the combinations in full, without '':matches()''.)
+		<li>
+			The specificity of an '':is()'' pseudo-class is replaced by 0.
 		<li>
 			Similarly, the specificity of an '':nth-child()'', '':nth-last-child()'', '':nth-of-type()'', or '':nth-last-of-type()'' selector
 		  is the specificity of the pseudo class itself (counting as one pseudo-class selector)
@@ -3571,6 +3615,7 @@ Changes</h2>
 	Significant changes since the <a href="https://www.w3.org/TR/2013/WD-selectors4-20130502/">2 May 2013 Working Draft</a> include:
 
 	<ul>
+		<li>Added '':is()'' pseudo-class.</li>
 		<li>Split out <a>relative selectors</a> from <a>scoped selectors</a>,
 			as these are different concepts that can be independently invoked.
 		<li>Changed rules for absolutizing a relative selector.