TIL there’s a .mml file extension for standalone MathML files (like there’s .svg for SVG, you know), and only Firefox supports MML rendering! Chrome shows it as plain text. Safari doesn’t even bother with that. Naturally, I filed feature requests, but I’m not holding my breath: it’s MathML, after all.
Safari https://bugs.webkit.org/show_bug.cgi?id=280682
Chrome https://issues.chromium.org/u/2/issues/370694822