The section in the article about fetching tiles will be of interest to anyone starting out in WebGL game development. If you've got server-side experience then designing the API for this is an early problem that you'll have to tackle:
A very simple and efficient way to determine the potentially visible tiles is by performing a simple intersection test against the bounding box of the viewing frustum's projection on the Y = 0 (horizontal) plane (see first image below).
A slightly more involved but potentially better way (as long as the viewpoint remains above and relatively close to the ground) is to construct our bounding box from the points of the polygon that results from intersecting the frustum with the horizontal plane, combined with the projected points of the frustum's near plane (got that?).
All in all, the GUI weighs in at ~12KLOC and spans 24 files, which is quite a bit more than the terrain engine itself!
It sounds like WebGL GUIs can be complicated, depending on your requirements. In this case I think it was well worth the effort.