Nice; thank you for making this available! I gave it a quick try on the current Firefox for Win10, with a few small (on the order of 160*100 mm²) two-layer boards. A few comments in no particular order:
Rendering was certainly correct with everything I tried. The only exception was a somewhat pathological design with a big bitmap in the silkscreen layer, where I lost patience after a few minutes.
It would be nice if it could respond a bit faster. Panning, zooming out, enabling/disabling layers. (This refers to the non-pathological boards I tried. Despite the small size, there was always a lag of a second or two -- enough to make you wonder whether the software had got your command, especially when selecting layers.)
The TypeScript implementation probably limits what you can do on that front; maybe you could a least give a preliminary indication that something is happening? E.g. when toggling layers on/off, toggle the checkbox immediately, then redraw. Or when panning or zooming, draw an outline immediately, the render the actual PCB? Zooming in is nice in this repsect, first shows the magnified low-res version, then adds high res when ready.
For the "pathological" bitmap mentioned above, I noticed that the rendering attempt slowed down the whole browser (other tabs), to the point of making it unusable. Probably due to the way Firefox implements TypeScript, hence not something you can influence? But it was really annoying and a bit worrying. (I didn't want to lose what I had typed here, for example.) So if you can work around this, it would be worth a try.
The actual rendered colors change in non-obvious ways. Copper layer alone is shown in red; when I add (white) silkscreen, the copper color changes to a strange, low contrast light brown or gold. Other common combinations, e.g. copper plus soldermask, also result in unexpected color changes.
The default choice of layer colors doesn't work for me. Top and bottom copper both show in the same color -- can't see much, with just those two layers enabled.
That's what struck my eye in the first test. Again, the rendering is nice, and the handling is straightforward -- I like your work! If you could tweak some of the aspects above, this could very well become my preferred Gerber viewer.
EDIT: Typos... I like your work!