The math alphabets such as \mathcal{ABCDEF} are rendered using <mi data-mjx-variant="-tex-calligraphic" mathvariant="script">A</mi> mathvariant is not in mathml-core so the end result depends on CSS and local font install. On Chrome I get Cambria Math ascii in italic style A and in firefox I get STix Two Math Ascii A in italic style, so not calligraphic/script in either case. It would be better to use the Unicode math alphabet codepoints, so in this case 𝒜ℬ𝒞𝒟ℰℱ”