Not really. Using JavaScript to add mask <div>'s is a messier solution, requiring more KB of code, especially if you want it to work on all browsers, even those that don't properly support DOM manipulation. Also the JavaScript would have to run after the other parent <div> has already been downloaded and rendered, meaning that there would be a brief time when there was no mask.

In my opinion, it is just easier and cleaner to add an extra <div> to the code than to add a bunch of extra lines of JavaScript to do the same thing.