I use a lot and tried to build a simple version for the browser (to use on windows). Not even remotely close to feature set but still works on basic stuffs.
I think Numi's more oriented towards a general calculator, whereas CalcuLaTeX is more for longer form problems or documents, although I know that at least a few people use it like a scratchpad. I'll definitely take some inspiration from Numi though.
By the way, the new website for CalcuLaTeX is https://calcula.tech