How can I overlay a div with semi-transparent opacity over a youtube iframe embedded video?

How can I overlay a div with semi-transparent opacity over a 

youtube iframe embedded video?

<iframe type="text/html"  

    src="http://www.youtube.com/embed/NWHfY_lvKIQ">

</iframe> 

Make sure its the first parameter in the URL. Other parameters must go after

Example :

<iframe type="text/html"   

  src="http://www.youtube.com/embed/NWHfY_lvKIQ?wmode=opaque">

</iframe>