find out if element is within the viewport, thus visible in that sense

js element is visible in viewport hittest

// http://ejohn.org/blog/getboundingclientrect-is-awesome/
// http://stackoverflow.com/questions/123999/how-to-tell-if-a-dom-element-is-visible-in-the-current-viewport/7557433#7557433

function visible_viewport(el, offset) {
	var r, html;
	offset = typeof offset !== 'undefined' ? offset : 200;
	if ( !el || 1 !== el.nodeType ) { return false; }
	html = document.documentElement;
	r = el.getBoundingClientRect();
	return ( !!r && r.bottom >= offset && r.right >= offset && r.top <= (html.clientHeight+offset) && r.left <= (html.clientWidth+offset) );
}