Hrm, sorry about Opera. I tested in a few browsers, but I confess my browser-dom-fu is weak.

From the look of it I'm telling MooTools to stop that event, but backspace doesn't seem to stop in Opera either. Maybe they don't want people disabling keyboard navigation maliciously?

edit: re: subset, my intention is for source-level 2.6 compatibility with the core language, but that's definitely far away.