For reference, here is a "simple" app - a parametric EQ designer that supports doing pink noise measurements of speakers (or headphones if you have the right equipment) with FFT in the browser: https://eq.spion.dev/ - works as an entirely static website :)

Typically what people say is "oh most websites won't need THAT" but the appetite for interactivity only grows with most companies, so you could easily end up in a slowly boiled frog scenario where your serverside HTML-JS mishmash is an absolute mess.