Good. The difficulty ramps up fast! I'm starting to guess on some somewhere around 6 or 7. (I haven't had my morning caffeine yet - I'm thinking I'll get to 10 later in the day.) So far only tested on Firefox / Fedora 36 (default/Gnome). No issues while I was playing, but while I was typing this, it "timed out", (10 fails in a row?) and now says my best streak was zero. Ripoff!
Part of me would like to see some sort of grouping/precedence assistance, but I guess that is part of the challenge. Format for MathJax - now THAT would be a programming challenge.