Plug your USB controller (Xbox, PlayStation, or Switch Pro) into your computer before opening your browser. Open your browser and load the game.
Because the developers do not make money from it. It is a non-commercial fan project. Playing it in a browser via emulation falls into a grey area. Historically, Nintendo targets downloadable launchers, not HTML5 browser games hosted on independent repos. However, this could change. Play it while you can. play super smash bros crusade in browser