Fuzzers are a tool that we’ve found incredibly valuable in the past, and continue to employ heavily. A fuzzer’s job is to make your application fail by feeding it surprising inputs. The good ones do this by knowing a part of your code well enough that they can make smart guesses about how to confuse it. This one, for instance, produces a constant stream of mostly-correct CSS rules, and watches to see whether the browser can cope with them. Because fuzzers take these random paths, they can uncover subtle bugs that are rarely encountered during “normal” testing; and Jesse is a master at building them.