@lxo See the references in my bug report:
https://bugzilla.mozilla.org/show_bug.cgi?id=1921407
⇒ the plan: https://bugzilla.mozilla.org/show_bug.cgi?id=1860492#c0
⇒ another bug report: https://bugzilla.mozilla.org/show_bug.cgi?id=1918930
Their suggestion to cope with that: ask websites to ship a non-standard player with emscripten-compiled javascript:
https://bugzilla.mozilla.org/show_bug.cgi?id=1921407#c6