From our TeX content survey, at #mobile-team > KaTeX survey results @ 💬, @rajveermalviya writes:

"""

469 messages failed because of unsupported css class: fix.
469 messages failed because of unsupported css class: inner.
447 messages failed because of unsupported css class: rlap.
444 messages failed because of unsupported css class: vbox.
444 messages failed because of unsupported css class: thinbox.

This seems to be a group of CSS classes used to render overlapped elements (e.g. $$ \ne $$).
"""

This was the 6th item after all the functionality included in releases so far (#46 (comment)).