Index: src/wp-content/themes/twentyseventeen/style.css
===================================================================
--- src/wp-content/themes/twentyseventeen/style.css	(revision 39226)
+++ 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/class-wp-customize-manager.php
===================================================================
--- src/wp-includes/class-wp-customize-manager.php	(revision 39226)
+++ src/wp-includes/class-wp-customize-manager.php	(working copy)
@@ -3449,7 +3449,7 @@
 		$this->add_setting( 'external_header_video', array(
 			'theme_supports'    => array( 'custom-header', 'video' ),
 			'transport'         => 'postMessage',
-			'sanitize_callback' => 'esc_url',
+			'sanitize_callback' => 'esc_url_raw',
 			'validate_callback' => array( $this, '_validate_external_header_video' ),
 		) );
 
Index: src/wp-includes/js/wp-custom-header.js
===================================================================
--- src/wp-includes/js/wp-custom-header.js	(revision 39226)
+++ src/wp-includes/js/wp-custom-header.js	(working copy)
@@ -1,147 +1,432 @@
-(function( window, settings ) {
+/* global YT */
+( function( window, settings ) {
 
+	var NativeHandler, YouTubeHandler;
+
+	// Fail gracefully in unsupported browsers.
 	if ( ! ( 'addEventListener' in window ) ) {
-		// Fail gracefully in unsupported browsers.
 		return;
 	}
 
-	function wpCustomHeader() {
-		var handlers = {
-			nativeVideo: {
-				test: function( settings ) {
-					var video = document.createElement( 'video' );
-					return video.canPlayType( settings.mimeType );
-				},
-				callback: nativeHandler
-			},
-			youtube: {
-				test: function( settings ) {
-					return 'video/x-youtube' === settings.mimeType;
-				},
-				callback: youtubeHandler
-			}
-		};
+	/**
+	 * Trigger an event.
+	 *
+	 * @param {Element} target HTML element to dispatch the event on.
+	 * @param {string} name Event name.
+	 */
+	function trigger( target, name ) {
+		var evt;
 
-		function initialize() {
-			settings.container = document.getElementById( 'wp-custom-header' );
+		if ( 'function' === typeof window.Event ) {
+			evt = new Event( name );
+		} else {
+			evt = document.createEvent( 'Event' );
+			evt.initEvent( name, true, true );
+		}
 
-			if ( supportsVideo() ) {
-				for ( var id in handlers ) {
-					var handler = handlers[ id ];
+		target.dispatchEvent( evt );
+	}
 
-					if ( handlers.hasOwnProperty( id ) && handler.test( settings ) ) {
-						handler.callback( settings );
+	/**
+	 * Create a custom header instance.
+	 *
+	 * @class CustomHeader
+	 */
+	function CustomHeader() {
+		this.handlers = {
+			nativeVideo: new NativeHandler(),
+			youtube: new YouTubeHandler()
+		};
+	}
 
-						// 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 );
-						}
+	CustomHeader.prototype = {
+		/**
+		 * Initalize the custom header.
+		 *
+		 * If the environment supports video, loops through registered handlers
+		 * until one is found that can handle the video.
+		 */
+		initialize: function() {
+			if ( this.supportsVideo() ) {
+				for ( var id in this.handlers ) {
+					var handler = this.handlers[ id ];
 
+					if ( 'test' in handler && handler.test( settings ) ) {
+						this.activeHandler = handler.initialize.call( handler, settings );
+
+						// Dispatch custom event when the video is loaded.
+						trigger( document, 'wp-custom-header-video-loaded' );
 						break;
 					}
 				}
 			}
-		}
+		},
 
-		function supportsVideo() {
+		/**
+		 * Determines if the current environment supports video.
+		 *
+		 * Themes and plugins can override this method to change the criteria.
+		 *
+		 * @return {boolean}
+		 */
+		supportsVideo: function() {
 			// Don't load video on small screens. @todo: consider bandwidth and other factors.
-			if ( window.innerWidth < settings.minWidth  || window.innerHeight < settings.minHeight ) {
+			if ( window.innerWidth < settings.minWidth || window.innerHeight < settings.minHeight ) {
 				return false;
 			}
 
 			return true;
+		},
+
+		/**
+		 * Base handler for custom handlers to extend.
+		 *
+		 * @type {BaseHandler}
+		 */
+		BaseVideoHandler: BaseHandler
+	};
+
+	/**
+	 * Create a video handler instance.
+	 *
+	 * @class BaseHandler
+	 */
+	function BaseHandler() {}
+
+	BaseHandler.prototype = {
+		/**
+		 * Initialize the video handler.
+		 *
+		 * @param {object} settings Video settings.
+		 */
+		initialize: function( settings ) {
+			var handler = this,
+				button = document.createElement( 'button' );
+
+			this.settings = settings;
+			this.container =  document.getElementById( 'wp-custom-header' ),
+			this.button = button;
+
+			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;
+
+			// Toggle video playback when the button is clicked.
+			button.addEventListener( 'click', function() {
+				if ( handler.isPaused() ) {
+					handler.play();
+				} else {
+					handler.pause();
+				}
+			});
+
+			// Update the button class and text when the video state changes.
+			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;
+			});
+
+			this.ready();
+		},
+
+		/**
+		 * Ready method called after a handler is initialized.
+		 *
+		 * @abstract
+		 */
+		ready: function() {},
+
+		/**
+		 * Whether the video is paused.
+		 *
+		 * @abstract
+		 * @return {boolean}
+		 */
+		isPaused: function() {},
+
+		/**
+		 * Pause the video.
+		 *
+		 * @abstract
+		 */
+		pause: function() {},
+
+		/**
+		 * Play the video.
+		 *
+		 * @abstract
+		 */
+		play: function() {},
+
+		/**
+		 * Append a video node to the header container.
+		 *
+		 * @param {Element} node HTML element.
+		 */
+		setVideo: function( node ) {
+			var editShortcutNode,
+				editShortcut = this.container.getElementsByClassName( 'customize-partial-edit-shortcut' );
+
+			if ( editShortcut.length ) {
+				editShortcutNode = this.container.removeChild( editShortcut[0] );
+			}
+
+			this.container.innerHTML = '';
+			this.container.appendChild( node );
+
+			if ( editShortcutNode ) {
+				this.container.appendChild( editShortcutNode );
+			}
+		},
+
+		/**
+		 * Show the video controls.
+		 *
+		 * Appends a play/pause button to header container.
+		 */
+		showControls: function() {
+			if ( ! this.container.contains( this.button ) ) {
+				this.container.appendChild( this.button );
+			}
+		},
+
+		/**
+		 * Whether the handler can process a video.
+		 *
+		 * @abstract
+		 * @param {object} settings Video settings.
+		 * @return {boolean}
+		 */
+		test: function() {
+			return false;
+		},
+
+		/**
+		 * Trigger an event on the header container.
+		 *
+		 * @param {string} name Event name.
+		 */
+		trigger: function( name ) {
+			trigger( this.container, name );
 		}
+	};
 
-		return {
-			handlers: handlers,
-			initialize: initialize,
-			supportsVideo: supportsVideo
-		};
-	}
+	/**
+	 * Create a custom handler.
+	 *
+	 * @param  {object} protoProps Properties to apply to the prototype.
+	 * @return CustomHandler The subclass.
+	 */
+	BaseHandler.extend = function( protoProps ) {
+		var prop;
 
-	function nativeHandler( settings ) {
-		var video = document.createElement( 'video' );
+		function CustomHandler() {
+			var result = BaseHandler.apply( this, arguments );
+			return result;
+		}
 
-		video.id = 'wp-custom-header-video';
-		video.autoplay = 'autoplay';
-		video.loop = 'loop';
-		video.muted = 'muted';
-		video.width = settings.width;
-		video.height = settings.height;
+		CustomHandler.prototype = Object.create( BaseHandler.prototype );
+		CustomHandler.prototype.constructor = CustomHandler;
 
-		video.addEventListener( 'click', function() {
-			if ( video.paused ) {
-				video.play();
+		for ( prop in protoProps ) {
+			CustomHandler.prototype[ prop ] = protoProps[ prop ];
+		}
+
+		return CustomHandler;
+	};
+
+	/**
+	 * Native video handler.
+	 *
+	 * @class NativeHandler
+	 */
+	NativeHandler = BaseHandler.extend({
+		/**
+		 * Whether the native handler supports a video.
+		 *
+		 * @param {object} settings Video settings.
+		 * @return {boolean}
+		 */
+		test: function( settings ) {
+			var video = document.createElement( 'video' );
+			return video.canPlayType( settings.mimeType );
+		},
+
+		/**
+		 * Set up a native video element.
+		 */
+		ready: function() {
+			var handler = this,
+				video = document.createElement( 'video' );
+
+			video.id = 'wp-custom-header-video';
+			video.autoplay = 'autoplay';
+			video.loop = 'loop';
+			video.muted = 'muted';
+			video.width = this.settings.width;
+			video.height = this.settings.height;
+
+			video.addEventListener( 'play', function() {
+				handler.trigger( 'play' );
+			});
+
+			video.addEventListener( 'pause', function() {
+				handler.trigger( 'pause' );
+			});
+
+			video.addEventListener( 'canplay', function() {
+				handler.showControls();
+			});
+
+			this.video = video;
+			handler.setVideo( video );
+			video.src = this.settings.videoUrl;
+		},
+
+		/**
+		 * Whether the video is paused.
+		 *
+		 * @return {boolean}
+		 */
+		isPaused: function() {
+			return this.video.paused;
+		},
+
+		/**
+		 * Pause the video.
+		 */
+		pause: function() {
+			this.video.pause();
+		},
+
+		/**
+		 * Play the video.
+		 */
+		play: function() {
+			this.video.play();
+		}
+	});
+
+	/**
+	 * YouTube video handler.
+	 *
+	 * @class YouTubeHandler
+	 */
+	YouTubeHandler = BaseHandler.extend({
+		/**
+		 * Whether the handler supports a video.
+		 *
+		 * @param {object} settings Video settings.
+		 * @return {boolean}
+		 */
+		test: function( settings ) {
+			return 'video/x-youtube' === settings.mimeType;
+		},
+
+		/**
+		 * Set up a YouTube iframe.
+		 *
+		 * Loads the YouTube IFrame API if the 'YT' global doesn't exist.
+		 */
+		ready: function() {
+			var handler = this;
+
+			if ( 'YT' in window ) {
+				YT.ready( handler.loadVideo.bind( handler ) );
 			} else {
-				video.pause();
+				var tag = document.createElement( 'script' );
+				tag.src = 'https://www.youtube.com/iframe_api';
+				tag.onload = function () {
+					YT.ready( handler.loadVideo.bind( handler ) );
+				};
+
+				document.getElementsByTagName( 'head' )[0].appendChild( tag );
 			}
-		});
+		},
 
-		settings.container.innerHTML = '';
-		settings.container.appendChild( video );
-		video.src = settings.videoUrl;
-	}
+		/**
+		 * Load a YouTube video.
+		 */
+		loadVideo: function() {
+			var handler = this,
+				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)?=))([^#\&\?]*).*/;
 
-	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];
+			video.id = 'wp-custom-header-video';
+			handler.setVideo( video );
 
-		function loadVideo() {
-			var YT = window.YT || {};
-
-			YT.ready(function() {
-				var video = document.createElement( 'div' );
-				video.id = 'wp-custom-header-video';
-				settings.container.innerHTML = '';
-				settings.container.appendChild( 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();
-							}
+			handler.player = new YT.Player( video, {
+				height: this.settings.height,
+				width: this.settings.width,
+				videoId: this.settings.videoUrl.match( VIDEO_ID_REGEX )[1],
+				events: {
+					onReady: function( e ) {
+						e.target.mute();
+						handler.showControls();
+					},
+					onStateChange: function( e ) {
+						if ( YT.PlayerState.PLAYING === e.data ) {
+							handler.trigger( 'play' );
+						} else if ( YT.PlayerState.PAUSED === e.data ) {
+							handler.trigger( '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();
-		} else {
-			var tag = document.createElement( 'script' );
-			tag.src = 'https://www.youtube.com/player_api';
-			tag.onload = function () { loadVideo(); };
-			document.getElementsByTagName( 'head' )[0].appendChild( tag );
+		/**
+		 * Whether the video is paused.
+		 *
+		 * @return {boolean}
+		 */
+		isPaused: function() {
+			return YT.PlayerState.PAUSED === this.player.getPlayerState();
+		},
+
+		/**
+		 * Pause the video.
+		 */
+		pause: function() {
+			this.player.pauseVideo();
+		},
+
+		/**
+		 * Play the video.
+		 */
+		play: function() {
+			this.player.playVideo();
 		}
-	}
+	});
 
+	// Initialize the custom header when the DOM is ready.
 	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 );
 
+	// Selective refresh support in the Customizer.
 	if ( 'customize' in window.wp ) {
 		wp.customize.selectiveRefresh.bind( 'render-partials-response', function( response ) {
 			if ( 'custom_header_settings' in response ) {
Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 39226)
+++ 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 ) ) {