WebGL is a 3D drawing context for the HTML5 canvas object, but what is not often mentioned is that it is fairly primitive. It is a low level 3D API based on OpenGL/ES and as such a lot of programmers have a good idea how to use it but.. what if you want to create 3D apps without having to grub about in the GPU basement?
At a slightly less exciting level, although essential, it includes methods for matrix and vector operations - something WebGL lacks. It also has support for easy 3D models complete with normals and textures and there are a small number of standard models such as a cube and a sphere. Utilities to make using worker threads easier are also provided along with methods to load resources.
There is documentation of the full API on the website and lots of examples to get you started - and some are quite impressive.
Overall the framework strikes a good balance between abstraction and staying reasonably close to the OpenGL way of doing things. Given that it is open source and issued under the MIT licence it is worth a try.
A Programmer's Guide to Canvas
Getting started with WebGL