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I don't think it's working for me (none of the keybinds work, and I don't see the fps counter). Brave, script-blocking disabled.

It is a little unclear. You have to click the "Try it in your browser!" link: https://shadow.goose.icu/

Did you click the demo?

The linked page has a screenshot of the engine. You have to click through to get the full viewport experience (with keybindings).