Snippet to keep reveal.js windows in sync using storage events.

revealjs-storage-sync.js

// only for top level windows
if(window.top === window){

  // store the new slide index
	Reveal.addEventListener('slidechanged', function(e) {
		var send = e.indexh + ',' + e.indexv;
		localStorage.setItem('rslide',send);
	});

	// restore 
	window.addEventListener("storage", function(e){
		if(e.key == 'rslide'){
			var vals = e.newValue.split(',').map(parseFloat);
			Reveal.slide(vals[0],vals[1]);
		}
	}, false);

}