Bug 7296 enabled exceptions for invalid CSS values being set via DOM. This has caused a problem with Mozilla-only cursors: Mozilla (of course) supports those, and IE has a quirk that lets it handle them without raising an exception. In IE, "-moz-grab" is treated as "url(-moz-grab)". Moreover, "url(foobar), -moz-grab" is also parsed "successfully" somehow. So, WebKit is the odd one raising an exception now. This can be fixed by adding support for those cursors (bug 8988) or by implementing an IE-style parsing quirk.
It was decided that exceptions shouldn't be raised for invalid values, so this is a non-issue.