I wrote a CSS grammar fuzzer to test Gecko's CSS parser. This fuzzer's tricks:
Declarative context-free grammar. This makes it easy to add new CSS features to the fuzzer, or even use it to test grammars other than CSS. Each symbol can be a star, concatenation, or list of alternatives. Unlike a parser, a fuzzer has to make decisions about what to create, so alternatives can be given weights and stars have a suggested number of repetitions. This alone was enough to find at least one bug in Gecko.
Breaking rules. Like any good fuzzer, it doesn't always follow the given context-free grammar. Sometimes it does weird stuff, such as inserting a random symbol, to throw the parser off. I was surprised that this only found one additional bug in Gecko. Perhaps this reflects the comprehensive error handling requirements of the CSS specifications and the corresponding test suites.
Grammatical recursion. When the fuzzer notices that a symbol is the same as an ancestor, it can repeat the parts of the final string between the two symbols. This is effective at finding bugs where large input can cause a recursive algorithm to run out of stack space and crash. This found four grammar-recursion crashes in Gecko.
None of these Gecko bugs seem to be security holes. I shared the fuzzer with other browser vendors privately for over a month, and nobody asked me to delay the release, so I believe it didn't find security holes in other browsers either. But I think this has more to do with CSS parsing being fairly simple and self-contained than any weakness in the fuzzer.