No idea, I've never tried. The engine is quite a performance hog and the HTML5 version of GM doesn't have super good performance since it's doubly interpreted, and you'd lose out on both hardware-accelerated 3D rendering and hardware lights. Also I'm pretty sure the way the mouse position is read to let you aim with the mouse just wouldn't work in HTML5 (since you can't control the mouse position other than on the desktop ports, and the code is based on resetting the mouse to the center of the screen all the time so you get motion from its absolute position).