Index: src/wp-content/themes/twentyseventeen/style.css
===================================================================
--- src/wp-content/themes/twentyseventeen/style.css	(revision 39203)
+++ src/wp-content/themes/twentyseventeen/style.css	(working copy)
@@ -4149,3 +4149,9 @@
 		padding: 0;
 	}
 }
+
+/* For testing purposes. */
+.wp-custom-header-video-button {
+	background: red;
+	position: relative;
+}
Index: src/wp-includes/js/wp-custom-header.js
===================================================================
--- src/wp-includes/js/wp-custom-header.js	(revision 39203)
+++ src/wp-includes/js/wp-custom-header.js	(working copy)
@@ -1,4 +1,5 @@
-(function( window, settings ) {
+/* globals YT:false */
+( function( window, settings ) {
 
 	if ( ! ( 'addEventListener' in window ) ) {
 		// Fail gracefully in unsupported browsers.
@@ -5,64 +6,109 @@
 		return;
 	}
 
-	function wpCustomHeader() {
-		var handlers = {
+	function trigger( target, name ) {
+		var evt;
+
+		if ( 'function' === typeof window.Event ) {
+			evt = new Event( name );
+		} else {
+			evt = document.createEvent( 'Event' );
+			evt.initEvent( name, true, true );
+		}
+
+		target.dispatchEvent( evt );
+	}
+
+	function CustomHeader() {
+		this.handlers = {
 			nativeVideo: {
 				test: function( settings ) {
 					var video = document.createElement( 'video' );
 					return video.canPlayType( settings.mimeType );
 				},
-				callback: nativeHandler
+				callback: NativeHandler
 			},
 			youtube: {
 				test: function( settings ) {
 					return 'video/x-youtube' === settings.mimeType;
 				},
-				callback: youtubeHandler
+				callback: YouTubeHandler
 			}
 		};
+	}
 
-		function initialize() {
-			settings.container = document.getElementById( 'wp-custom-header' );
+	CustomHeader.prototype.initialize = function() {
+		var activeHandler,
+			button = document.createElement( 'button' );
 
-			if ( supportsVideo() ) {
-				for ( var id in handlers ) {
-					var handler = handlers[ id ];
+		button.setAttribute( 'type', 'button' );
+		button.setAttribute( 'id', 'wp-custom-header-video-button' );
+		button.setAttribute( 'class', 'wp-custom-header-video-button wp-custom-header-video-play' );
+		button.setAttribute( 'aria-hidden', true );
+		button.innerHTML = settings.l10n.play;
 
-					if ( handlers.hasOwnProperty( id ) && handler.test( settings ) ) {
-						handler.callback( settings );
+		this.container = document.getElementById( 'wp-custom-header' ),
+		this.button = button;
 
-						// Set up and dispatch custom event when the video is loaded.
-						if ( 'dispatchEvent' in window ) {
-							var videoLoaded = new Event( 'wp-custom-header-video-loaded' );
-							document.dispatchEvent( videoLoaded );
-						}
+		if ( this.supportsVideo() ) {
+			for ( var id in this.handlers ) {
+				var handler = this.handlers[ id ];
 
-						break;
-					}
+				if ( this.handlers.hasOwnProperty( id ) && handler.test( settings ) ) {
+					activeHandler = handler.callback.call( this, this, settings );
+
+					// Dispatch custom event when the video is loaded.
+					trigger( document, 'wp-custom-header-video-loaded' );
+					break;
 				}
 			}
+
+			this.container.addEventListener( 'play', function() {
+				button.className = 'wp-custom-header-video-button wp-custom-header-video-play';
+				button.innerHTML = settings.l10n.pause;
+			});
+
+			this.container.addEventListener( 'pause', function() {
+				button.className = 'wp-custom-header-video-button wp-custom-header-video-pause';
+				button.innerHTML = settings.l10n.play;
+			});
+
+			button.addEventListener( 'click', function() {
+				if ( activeHandler.paused() ) {
+					activeHandler.play();
+				} else {
+					activeHandler.pause();
+				}
+			});
 		}
+	};
 
-		function supportsVideo() {
-			// Don't load video on small screens. @todo: consider bandwidth and other factors.
-			if ( window.innerWidth < settings.minWidth  || window.innerHeight < settings.minHeight ) {
-				return false;
-			}
+	CustomHeader.prototype.supportsVideo = function() {
+		// Don't load video on small screens. @todo: consider bandwidth and other factors.
+		if ( window.innerWidth < settings.minWidth || window.innerHeight < settings.minHeight ) {
+			return false;
+		}
 
-			return true;
+		return true;
+	};
+
+	CustomHeader.prototype.setButton = function() {
+		if ( ! this.container.contains( this.button ) ) {
+			this.container.appendChild( this.button );
 		}
+	};
 
-		return {
-			handlers: handlers,
-			initialize: initialize,
-			supportsVideo: supportsVideo
-		};
-	}
+	CustomHeader.prototype.setVideo = function( node ) {
+		this.container.innerHTML = '';
+		this.container.appendChild( node );
+	};
 
-	function nativeHandler( settings ) {
+	function NativeHandler( header, settings ) {
+		if ( ! ( this instanceof NativeHandler ) ) {
+			return new NativeHandler( header, settings );
+		}
+
 		var video = document.createElement( 'video' );
-
 		video.id = 'wp-custom-header-video';
 		video.autoplay = 'autoplay';
 		video.loop = 'loop';
@@ -70,77 +116,111 @@
 		video.width = settings.width;
 		video.height = settings.height;
 
-		video.addEventListener( 'click', function() {
-			if ( video.paused ) {
-				video.play();
-			} else {
-				video.pause();
-			}
+		video.addEventListener( 'play', function() {
+			trigger( header.container, 'play' );
 		});
 
-		settings.container.innerHTML = '';
-		settings.container.appendChild( video );
+		video.addEventListener( 'pause', function() {
+			trigger( header.container, 'pause' );
+		});
+
+		video.addEventListener( 'canplay', function() {
+			header.setButton();
+		});
+
+		header.setVideo( video );
 		video.src = settings.videoUrl;
+
+		return {
+			play: video.play.bind( video ),
+
+			pause: video.pause.bind( video ),
+
+			paused: function() {
+				return video.paused;
+			}
+		};
 	}
 
-	function youtubeHandler( settings ) {
-		// @link http://stackoverflow.com/a/27728417
-		var VIDEO_ID_REGEX = /^.*(?:(?:youtu\.be\/|v\/|vi\/|u\/\w\/|embed\/)|(?:(?:watch)?\?v(?:i)?=|\&v(?:i)?=))([^#\&\?]*).*/,
-			videoId = settings.videoUrl.match( VIDEO_ID_REGEX )[1];
+	function YouTubeHandler( header, settings ) {
+		if ( ! ( this instanceof YouTubeHandler ) ) {
+			return new YouTubeHandler( header, settings );
+		}
 
+		var player;
+
 		function loadVideo() {
-			var YT = window.YT || {};
+			var video = document.createElement( 'div' ),
+				// @link http://stackoverflow.com/a/27728417
+				VIDEO_ID_REGEX = /^.*(?:(?:youtu\.be\/|v\/|vi\/|u\/\w\/|embed\/)|(?:(?:watch)?\?v(?:i)?=|\&v(?:i)?=))([^#\&\?]*).*/;
 
-			YT.ready(function() {
-				var video = document.createElement( 'div' );
-				video.id = 'wp-custom-header-video';
-				settings.container.innerHTML = '';
-				settings.container.appendChild( video );
+			video.id = 'wp-custom-header-video';
+			header.setVideo( video );
 
-				new YT.Player( video, {
-					height: settings.height,
-					width: settings.width,
-					videoId: videoId,
-					events: {
-						onReady: function( e ) {
-							e.target.mute();
-						},
-						onStateChange: function( e ) {
-							if ( YT.PlayerState.ENDED === e.data ) {
-								e.target.playVideo();
-							}
+			player = new YT.Player( video, {
+				height: settings.height,
+				width: settings.width,
+				videoId: settings.videoUrl.match( VIDEO_ID_REGEX )[1],
+				events: {
+					onReady: function( e ) {
+						e.target.mute();
+						header.setButton();
+					},
+					onStateChange: function( e ) {
+						if ( YT.PlayerState.PLAYING === e.data ) {
+							trigger( header.container, 'play' );
+						} else if ( YT.PlayerState.PAUSED === e.data ) {
+							trigger( header.container, 'pause' );
+						} else if ( YT.PlayerState.ENDED === e.data ) {
+							e.target.playVideo();
 						}
-					},
-					playerVars: {
-						autoplay: 1,
-						controls: 0,
-						disablekb: 1,
-						fs: 0,
-						iv_load_policy: 3,
-						loop: 1,
-						modestbranding: 1,
-						//origin: '',
-						playsinline: 1,
-						rel: 0,
-						showinfo: 0
 					}
-				});
+				},
+				playerVars: {
+					autoplay: 1,
+					controls: 0,
+					disablekb: 1,
+					fs: 0,
+					iv_load_policy: 3,
+					loop: 1,
+					modestbranding: 1,
+					playsinline: 1,
+					rel: 0,
+					showinfo: 0
+				}
 			});
 		}
 
 		if ( 'YT' in window ) {
-			loadVideo();
+			YT.ready( loadVideo );
 		} else {
 			var tag = document.createElement( 'script' );
-			tag.src = 'https://www.youtube.com/player_api';
-			tag.onload = function () { loadVideo(); };
+			tag.src = 'https://www.youtube.com/iframe_api';
+			tag.onload = function () {
+				YT.ready( loadVideo );
+			};
+
 			document.getElementsByTagName( 'head' )[0].appendChild( tag );
 		}
+
+		return {
+			play: function() {
+				player.playVideo();
+			},
+
+			pause: function() {
+				player.pauseVideo();
+			},
+
+			paused: function() {
+				return YT.PlayerState.PAUSED === player.getPlayerState();
+			}
+		};
 	}
 
 	window.wp = window.wp || {};
-	window.wp.customHeader = new wpCustomHeader();
-	document.addEventListener( 'DOMContentLoaded', window.wp.customHeader.initialize, false );
+	window.wp.customHeader = new CustomHeader();
+	document.addEventListener( 'DOMContentLoaded', window.wp.customHeader.initialize.bind( window.wp.customHeader ), false );
 
 	if ( 'customize' in window.wp ) {
 		wp.customize.selectiveRefresh.bind( 'render-partials-response', function( response ) {
Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 39203)
+++ src/wp-includes/theme.php	(working copy)
@@ -1381,6 +1381,10 @@
 		'height'    => absint( $header->height ),
 		'minWidth'  => 900,
 		'minHeight' => 500,
+		'l10n'      => array(
+			'pause' => __( 'Pause' ),
+			'play'  => __( 'Play' ),
+		),
 	);
 
 	if ( preg_match( '#^https?://(?:www\.)?(?:youtube\.com/watch|youtu\.be/)#', $video_url ) ) {