in js/reveal.js [1458:1495]
function showPreview( url ) {
closeOverlay();
dom.overlay = document.createElement( 'div' );
dom.overlay.classList.add( 'overlay' );
dom.overlay.classList.add( 'overlay-preview' );
dom.wrapper.appendChild( dom.overlay );
dom.overlay.innerHTML = [
'<header>',
'<a class="close" href="#"><span class="icon"></span></a>',
'<a class="external" href="'+ url +'" target="_blank"><span class="icon"></span></a>',
'</header>',
'<div class="spinner"></div>',
'<div class="viewport">',
'<iframe src="'+ url +'"></iframe>',
'</div>'
].join('');
dom.overlay.querySelector( 'iframe' ).addEventListener( 'load', function( event ) {
dom.overlay.classList.add( 'loaded' );
}, false );
dom.overlay.querySelector( '.close' ).addEventListener( 'click', function( event ) {
closeOverlay();
event.preventDefault();
}, false );
dom.overlay.querySelector( '.external' ).addEventListener( 'click', function( event ) {
closeOverlay();
}, false );
setTimeout( function() {
dom.overlay.classList.add( 'visible' );
}, 1 );
}