Oh, this is fun. Rather different experience from just playing on a regular emulator.
UX: Overall good, I'd say. It's kind of a shame that you don't end up interacting with the physical object too much - you can click on the buttons, but you'll mostly be using your keyboard. Gamepad controls would be great, but those are more "nice to have" than a must.
WebGL performance: Seemed pretty fine. I had a lot of tabs open, including one with a YouTube video, and was able to play the GameBoy Colour game at high quality with no performance issues.
Keyboard configuration: I live in a country where keyboards have a QWERTZ layout, not QWERTY, so some layouts were inconvenient. (Notably the GameBoy, which expects "X" and "Z" to be adjacent.) It wasn't a big deal because there were alternate keys included, though.