From docs: > Web mining is infeasible due to the large memory requirement and the lack of directed rounding support for floating point operations in both Javascript and WebAssembly.
So you can do whatever you want, but you will end with nothing.
Still. I am inclined to call that "impractical". In the same way softfloat-in-JS miners are impractical. A vast botnet of such might buy a coffee after a year or two.
At a certain point, slow enough turns into impractical, and then practically impossible.
[1] http://www.righto.com/2015/05/bitcoin-mining-on-55-year-old-...
That is, after all, the whole idea behind asymmetric cryptography. You could perhaps crack some things by brute forcing until the heat death of the universe, but that's so slow as to be impossible for all practical purposes.