Probably not enough RAM. The page isn't doing anything crazy, but includes a dozen or so example <video>'s, which in turn can send Linux into an out-of-memory situation that freezes the system (technically it's still working just really slow). Ran into that issue a lot when browsing around with 8GB RAM, upgrading that helped. Installing earlyoom[1] is another workaround.
Thanks, earlyoom looks interesting. But as I know where my memory usage goes (Firefox, mostly) I think I'll just write a script to kill Firefox if I run out. Much better than waiting for the scheduler to kill KDE out from under me!