No... I actually ported SFML to use non-legacy GL first then ported that to emscripten. It uses the "WebGL-friendly subset of OpenGL" (i.e. OpenGL ES 2.0 without client-side arrays a.k.a. only VBOs) that yields the highest performance when translated to WebGL.
The emscripten port is not complete yet because there is currently no way to implement shared-memory multi-threading and this is why besides sf::Thread things that rely on threads like sf::Music can't work yet. According to the emscripten guys they are also waiting for JavaScript memory sharing between threads to be standardized so that they can implement pthread API emulation.