You opened this as a local file. YouTube embeds can return Error 153 from
file:// because there is no normal HTTP referrer.
Put this HTML in a folder and run python -m http.server 8000, then open http://localhost:8000/rhythm_editor_canvas_prototype_v15.html.
Load a YouTube URL to preview the music video here.
The centered button and Space both play / pause.
The centered button and Space both play / pause.
Use Refresh to load shared charts from /api/charts.