Created attachment 358679 [details] testcase Firefox's CSS parser doesn't mind the missing ")" as long as there's an EOF right there: @namespace url(x is accepted and turned into @namespace url(x); (I'm just guessing this is a bug. Safari and Opera don't seem to support .cssRules for @namespace, so it's hard for me to tell what they do.)
Created attachment 358680 [details] testcase 2 Similarly, Firefox seems fine with an unclosed quote at the end of the file: @namespace '
I found this more through accident than through fuzzing. If this is a bug, I can have the fuzzer test for some invariant like "adding \n at the end shouldn't affect the result of parsing" to look for more bugs like it.
We're supposed to be ok with these, I think, although it's a relatively recent change in the spec.
In particular, see http://www.w3.org/TR/CSS21/syndata.html#parsing-errors -- the parts about "Unexpected end of style sheet" and "Unexpected end of string".