It is rendering using the 2D canvas API. There's a device context class (wxDC) for each wxWidgets port that implements primitive drawing ops (DrawLine, DrawText, etc). For the WASM port, I created a glue library in javascript [1] that wxDC calls into for each primitive.
[1] https://github.com/ahilss/wxWidgets-wasm/blob/master/build/w...