Boolean attributes

Some HTML user agents are unable to interpret boolean attributes when these appear in their full (non-minimized) form, as required by XML 1.0. Note this problem doesn't affect user agents compliant with HTML 4. The following attributes are involved: compact, nowrap, ismap, declare, noshade, checked, disabled, readonly, multiple, selected, noresize, defer.

So what happens when you try to mess with the standard? :-)

<input type="checkbox" checked="unchecked">:

Invalid of course, but both Firefox and Opera check the checkbox by default.