So I know this because I did some research on why the crunchyroll subs didn't work in the pip mode on firefox. So it turns out they used a substitute format called .ass, which as the article mentioned was created by AegisSubs. This is not natively supported by browsers, so they used to import a wasm bundle for reading this file, rendering it on a Canvas that was overlayed on the Video. So they did put a lot of effort in the making it work not only in the labour of the translation but also in supporting it technically. Sadly it looks like they will be switching to more lackluster formats That don't support the advance positioning features.