Very recent is david a roberts work to get this running on Pyodide, marimo and Observable. See https://mathics3.github.io/Mathics3-live/ and https://github.com/Mathics3/Mathics3-notebook-frontends

All of this is a little new and wobbly, but I expect things to stabilize in a month or so. (And sooner or better if more people get involved).