When used with OpenGL SDL is responsible for opening a window, creating an opengl context, handling events, etc. The drawing part is OpenGL responsibility.
In a browser, you already have a window, ways to handle events and a way to get a context, SDL would not be very helpful in this scenario.
I think it's highly unlikely that SDL + WebGL is a useful combination.
Also the level of abstraction is fundamentally different: SDL was originally conceived as a small layer to allow native C programs to gain access to Framebuffers in an OS independent and convenient way. This allows games which use software rendering to be written more easily.
You're probably better off using something like Three.js if you are targetting a web browser (NB: do not consider this a recommendation)