It would be valuable to gather some URL-keyed data about WebGL's usage in the wild. A few initial metrics that would be useful:

1) Creation of a WebGL context, measuring 1.0 and 2.0 contexts separately.
2) Fetching and use of the WEBGL_debug_renderer_info extension (specifically, whether the UNMASKED_VENDOR_WEBGL and UNMASKED_RENDERER_WEBGL parameters are queried).
3) Whether the context did a certain number of draw calls.

Please suggest more. This initial data would already tell us whether WebGL is actually being used to draw graphics, vs. other purposes.