[selectors-4] Add :fullscreen and :picture-in-picture w3c#3796

Author: fantasai

selectors-4/Overview.bs

@@ -2434,7 +2434,7 @@ The Future-element Pseudo-class: '':future''</h3>
 
 
 <h2 id='resource-pseudos'>
-Resource State Pseudos</h2>
+Resource State Pseudo-classes</h2>
 
 	The pseudo-classes in this section apply to elements that represent loaded resources,
 	particularly images/videos,
@@ -2498,6 +2498,9 @@ 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=].
 
+<h2 id="display-state-pseudos">
+Element Display State Pseudo-classes</h2>
+
 <h3 id="modal-state">
 Modal (Exclusive Interaction) State: the '':modal'' pseudo-class</h3>
 
@@ -2507,6 +2510,32 @@ Modal (Exclusive Interaction) State: the '':modal'' pseudo-class</h3>
 	Multiple elements can be '':modal'' simultaneously,
 	with only one of them active (able to receive input).
 
+	<div class="example">
+		For example, the <{dialog}> element is '':modal''
+		when opened with the {{HTMLDialogElement/showModal()}} API.
+		Similarly, a '':fullscreen'' element is also '':modal''
+		when opened with the {{requestFullscreen()}} API,
+		since this prevents interaction with the rest of the page.
+	</div>
+
+<h3 id="fullscreen-state">
+Fullscreen Presentation State: the '':fullscreen'' pseudo-class</h3>
+
+	The <dfn>:fullscreen</dfn> pseudo-class represents
+	an element which is displayed in a mode that
+	takes up most (usually all) of the screen,
+	such as that defined by the Fullscreen API. [[FULLSCREEN]]
+
+<h3 id="pip-state">
+Picture-in-Picture Presentation State: the '':picture-in-picture'' pseudo-class</h3>
+
+	The <dfn>:picture-in-picture</dfn> pseudo-class represents
+	an element which is displayed in a mode that
+	takes up most (usually all) of the viewport,
+	and whose viewport is confined to part of the screen
+	while being displayed over other content,
+	for example when using the Picture-in-Picture API. [[picture-in-picture]]
+
 <h2 id='input-pseudos'>
 The Input Pseudo-classes</h2>