This is a spinoff from bug 173767. The hint when going from "color: red" to "color: red !important" is NONE, so no update (eg repaint) is done...
testcase provided in the url field ..... http://bugzilla.mozilla.org/attachment.cgi?id=104372&action=view
The current nsCSSDeclaration data storage makes this kinda hard to fix... :(
OK. We now generate the right hint.... but we don't reresolve style on anything because the assumption is that changes to a style rule's declaration don't change what order rules apply in. Bad assumption, when declarations get set to !important.... I suspect that the easiest way to fix this bug is via bug 188803.
Fixed by changes for bug 188803