How can I use my Blender models in WebGL I would like to create WebGL applications and use Blender as a modelling tool. I don't necessarily need to translate game engine functionality to WebGL but I would like to retain as much of the UV mapping, vertex colors, materials, etc. info as possible.

I already know I should keep the polygon count low. What are the other things I should keep an eye on? (Such as don't use feature X because you won't be able to export it)

What tools do I need to learn? (WebGL framework specific plugins, generic tools like OBJ exporter)
The answer is somewhat dependent on what file format the WebGL viewer ingests. And how good the Blender exporter is. But the most common format is probably the ASCII Obj format, for which Blender has a pretty decent exporter for.

So, things to watch out for:

Geometry: Avoid Ngons, or at least, triangulate before exporting. The exporter might produce unexpected results. Also, make sure the face normals are all oriented correctly. Blender renders both side of a polygon by default, many other 3D viewers don't. You can recalculate normals with Ctrl + N.

Mesh Modifiers: Even though the exporter probably applies the modifiers. It's better to be explicit than implicit. Also, it is a good idea to apply the transforms (Ctrl + A).

Materials and Textures: Avoid the Blender material panel(except basic colors). Use UV-mapped textures. Blender-specific features like procedural textures are not going to migrate properly. Image textures dimensions should be power of 2 square. (1024x1024, 512x512, etc)