[selectors-4] Add :modal. w3c#6965

Author: fantasai

selectors-4/Overview.bs

@@ -2491,6 +2491,15 @@ Sound State: the '':muted'' and '':volume-locked'' pseudo-classes</h3>
 	when programmatically changing the element’s volume
 	does not change the element's [=HTMLMediaElement/effective media volume=].
 
+<h3 id="modal-state">
+Modal (Exclusive Interaction) State: the '':modal'' pseudo-class</h3>
+
+	The <dfn>:modal</dfn> pseudo-class represents
+	an element which is in a state that excludes all interaction
+	with elements outside it until it has been dismissed.
+	Multiple elements can be '':modal'' simultaneously,
+	with only one of them active (able to receive input).
+
 <h2 id='input-pseudos'>
 The Input Pseudo-classes</h2>
 
@@ -4016,6 +4025,8 @@ Changes since the 21 November 2018 Working Draft</h3>
 		<li>Added [[#sub-pseudo-elements]] to define [=sub-pseudo-elements=] and related terminology.
 		<li>Added '':defined''.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/2258">Issue 2258</a>)
+		<li>Added '':modal''.
+			(<a href="https://github.com/w3c/csswg-drafts/issues/6965">Issue 6965</a>)
 		<li>Added '':seeking'', '':buffering'', and ':stalled'' media playback state pseudo-classes.
 			(<a href="https://github.com/w3c/csswg-drafts/issues/3821">Issue 3821</a>)
 		<li>Added '':muted'' and '':volume-locked'' sound state pseudo-classes.