fluid iframe (used for youtube vids)

css.css

.fluidVidContainer{width:100%;position:relative;margin-bottom:20px}
.fluidVidContainer:before{
  display:block;content:'';
  padding-top:56.25%;/*16x9*/
  padding-top:75%;/*4x3*/
  padding-top:200%;/*1x2*/
  padding-top:50%;/*2x1*/
  padding-top:100%;/*1x1*/
}
.fluidVidContainer iframe{position:absolute;top:0;left:0;width:100%;height:100%}

html.html

<iframe width="302" height="315" src="//www.youtube.com/embed/ScMzIvxBSi4" frameborder="0" allowfullscreen></iframe>

js.js

//requires helpers.js
if(zQ('iframe')){
	for(i=0;i<zQA('iframe').length;i++){
		var tag=zQA('iframe')[i];
		var container=document.createElement('div');
		zAddClass(container,'fluidVidContainer');
		if(!zHasClass(tag.parentNode,'fluidVidContainer')){
			wrap(tag,container);
		}
	}
}