Impressive as the emulator is, it's been suggested to him before (and he has acknowledged) that he shouldn't be running non-public-domain ROMs on his site. If he gets a C&D/DMCA takedown, I have zero sympathy.
Why does everything have to be on the web? As with most other things, a mixed approach is the best one.
Works far better than some other GBC emulators I've seen though, nice work.
I'd love to see an analysis of how the different major browsers handle this emulator.
Firefox 4+ runs it overall well (Slight GC hiccups in Firefox 9 though).
Opera 11.6 can't run it, because it doesn't follow the typed array spec, causing the emulated cpu to crash. Works in older opera versions though, but just a little faster than IE9.
Safari 5.1.X reloads the page when ever a game starts, because of a js engine regression that happened in safari 5.1.1. Works in webkit nightlies though, and Safari 5.0.X
IANAL, though, so if the person who made this gets any nastygrams, they should talk to someone who is.
That said, I wouldn't do what they're doing.