Basically emscripten provides a set of libraries that emulate C libraries in Javascript. For example you can use the standard Berkeley socket API, but it actually translates TCP and UDP into SCTP (which can support TCP-like and UDP-like modes) via WebRTC's DataChannel.
Similarly, emscripten provides a standard C OpenGL API, but it actually translates those calls to WebGL Javascript calls.
Similarly, emscripten provides a standard C OpenGL API, but it actually translates those calls to WebGL Javascript calls.