with regard to the general semantic overlook of the thing, why is this necessary?  if you want to know what a link is, you click it.  if you dont like what your seeing when you click it, click the back button.  

:)

that being said, are you talking about a quicklook functionality like in iOS or similar?  if so they should all do that by default on long press and hold of a link..

if its a request for a preview on hover, thats a java script type thing..  :)  and i dont know how, but gander it uses some kind of http call and fancybox to generate the preview which would be stored in a temp file. or something similar