The spec is here: http://dev.w3.org/html5/spec-author-view/dnd.html#the-dropzone-attribute It sounds like Opera already implements it. If their implementation matches WebKit's implementation, we should drop the prefix.
I'm not sure if we need to keep the webkit prefixed version for backwards compat. Do we know of any websites that use this?
Also see https://bugzilla.mozilla.org/show_bug.cgi?id=723008.
Blink Commit: https://chromium.googlesource.com/chromium/src.git/+/b152fa718af332bde2004d61e5b0ecbef9653ec1 & https://chromium.googlesource.com/chromium/src.git/+/ed2c34a329612b1c26912b9ad6329164dd481f6e
TL;DR: Gecko refused to implement and Chromium removed it. We should probably remove it too, but I suspect we might have to keep it around for embedders.
<rdar://problem/122588147>