echidna CSS validation issues

Reported at https://github.com/w3c/echidna/issues/271

Bug: CHTML has a typo ".mjx-span": {display:"span"} https://github.com/mathjax/MathJax/blob/d4ab1b35c96dd964eaa9e1ed2c86e39fffbdacf6/unpacked/jax/output/CommonHTML/jax.js#L126.

Not strictly a bug: but the menu still uses DXImageTransform etc (which are strictly speaking invalid). Not sure if we want to drop them but I feel like we could.