Last modified: 2014-09-03 15:00:38 UTC
The same setupEmptyDisabler() code that enables the submit button (e.g. [Reply] turns green) when you enter something in a textarea, should also add mw-ui-destructive to the Cancel button. I.e., the Cancel button should appear red on hover because you'll be losing your entry. See "Cancel button" discussion on design mailing list.
No longer relevant. Please file issue against MediaWiki/MediaWiki UI if something is not working correctly.