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Trident, Presto and Gecko all support mouseenter and mouseleave.
Also, these need to be [LenientThis]. See https://bugzilla.mozilla.org/show_bug.cgi?id=691059
This is just about having event handler attributes, right?
Yes
Checked in as WHATWG revision r7681. Check-in comment: Compatibility with deployed content. http://html5.org/tools/web-apps-tracker?from=7680&to=7681