a simple way would be just to place your html blocks into blank pages then call an iframe randomly with javascript
like place your random html into seperate html pages
random0.html
thru
random4.html
Code:
<script>
var rn= Math.floor(Math.random()*5);
document.write("<iframe width=800 height=100 src=random" + rn + ".html></iframe>");
</script>