In fact, this is only possible with CSS. In your smaller white backgound element, you can use CSS3 with fallback for IE 6-8. For backup you can use transparent PNG.
CSS3 code for white bg will be:
background-color: rgba(255,255,255,0.5)
I installed the code to view the code in action. http://codepen.io/DheerG/pen/tkKqC
Also, if you want everything in the smaller element to be transparent, you could just use the css opacity element.
opacity: 0.6; -ms-filter: "progid:DXImageTransform.Microsoft.Alpha(Opacity=60)"; filter: alpha(opacity=60);
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