WebAssembly has no access to the outside world at the moment, that is correct. It is only able to call (and be called by) JS.

A C++ application compiled via Emscripten ships (a fairly large amount) of JS glue code that exposes all relevant Browser APIs like WebGL, Fetch or other HTML5 stuff to the actual WASM program. As others commented, for WebGL an additional API translation is applied. If the source targets OpenGL ES 2 (or 3 for WebGL 2), this step has almost no overhead however.