Test for the Billion Laughs DoS attack.
Most browsers fix this... but here is a handy program to find ones that do not.
The server will return offensive XML by default at any request. Navigating to
/lol.html serves an HTML page to see if
DOMParser can choke on it.
As of this writing, the Firefox
DOMParser appears to be effected. (Cheers to @davejohnson for finding that!)