Thanks for this suggestion! My browser experiments so far were focused on Web3D (and the X3D format to store 3D models).
I understand that WebGL is similar to OpenGL as a low-level graphics API, targeted at the HTML5 canvas. But like with "pure" OpenGL ... it doesn't come with an event loop and GUI toolkit. That's what I use Tk for, and what I would have to rewrite for the HTML canvas.
Also, being based on OpenGL ES which is "a well-defined subset of desktop OpenGL suitable for low-power devices", it seems to lack the interactive glRenderMode feature which I need to use. Or maybe I just failed to find it?