The heightmaps are using in the WebGl???
As with OpenGL, there is no specific support for heightmaps - that's what you write shader code for.
The tricky part is that most heightmap implementations require either geometry shader support (which WebGL does not have) - or at the very least, the ability to use vertex textures.
Vertex textures are supported by WebGL - but the specification allows implementations to specify a maximum number of supported maps - which can legally be zero. Hence, at least a couple of drivers that I've seen do not support even vertex texturing.
Without that, you'll have to do your height mapping on the CPU side - or fall back to bump-mapping or normal-mapping on machines without vertex texture support.
Ok, so I'll investigate this type of mapping.