This is a followup to bug 1089326. The other kind of elements other than <button> that use this frame are input[type="reset|submit|button"], via nsGfxButtonControlFrame. And that frame is a leaf, so it can only contain anonymous content (which is chromeonly content and thus doesn't show up in event.target and co.). This changes behavior for event.originalTarget, I suppose, but that's what we do elsewhere as well, and that's a Gecko-specific thing.

Pushed by ealvarez@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/587615dad57e Remove unneeded special case in nsHTMLButtonControlFrame. r=bzbarsky