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

"""

444 messages failed because of unsupported css class: nulldelimiter.

This seems to be just a spacer according to its CSS definition. So, it should be straightforward to implement.
"""

I believe an example where this appears is in $$ \frac 1 2 x $$ ($$\frac 1 2 x$$), on either side of the fraction.

This was the 7th item after all the functionality included in releases so far (#46 (comment)).