startEmbeddedIframe( event ) {

		let iframe = event.target;

		if( iframe && iframe.contentWindow ) {

			let isAttachedToDOM = !!closest( event.target, 'html' ),
				isVisible  		= !!closest( event.target, '.present' );

			if( isAttachedToDOM && isVisible ) {

				// Prefer an explicit global autoplay setting
				let autoplay = this.Reveal.getConfig().autoPlayMedia;

				// If no global setting is available, fall back on the element's
				// own autoplay setting
				if( typeof autoplay !== 'boolean' ) {
					autoplay = iframe.hasAttribute( 'data-autoplay' ) || !!closest( iframe, '.slide-background' );
				}

				// YouTube postMessage API
				if( /youtube\.com\/embed\//.test( iframe.getAttribute( 'src' ) ) && autoplay ) {
					iframe.contentWindow.postMessage( '{"event":"command","func":"playVideo","args":""}', '*' );
				}
				// Vimeo postMessage API
				else if( /player\.vimeo\.com\//.test( iframe.getAttribute( 'src' ) ) && autoplay ) {
					iframe.contentWindow.postMessage( '{"method":"play"}', '*' );
				}
				// Generic postMessage API
				else {
					iframe.contentWindow.postMessage( 'slide:start', '*' );
				}

			}

		}

	}
"Starts" the content of an embedded iframe using the
postMessage API.

@param {object} event