It's not just that... It's pixels, em, ex, %, px, cm, mm, in, pt, pc... Not to mention margins, padding, box-model, absolute and relative positioning, flex-box, etc. etc. HTML/CSS is one of the most complex standards. You have to remove a lot before things get trivial.
There are already pretty advanced DAWs and video editors in the browser, so the UI isn’t the problem.