ever get annoyed af by a YouTube video's totally wrong aspect ratio? i do frequently. but i want to share a wonderful little tool someone made to fix it on the fly.
this bookmarklet by Alexander Pruss (github) is a javascript snippet that you set as a bookmark in your browser. when you invoke it, either by clicking it in the bookmark bar or by typing the keyword you assigned to it into the URL field, a window pops up asking for an aspect ratio. type one, hit OK, and it will then try to apply that AR to any HTML5 videos it detects on the page. this works on a variety of websites too, not just YT.
for example, i have the bookmarklet set to the keyword "rat", as in ratio. so whenever i run into a messed up video i just type rat in the URL field, press enter, type 4:3 or 16:9 or whatever into the box that appeared, and press enter again. in most cases the video will be fixed.
now you can enjoy the video without wasting brain cycles getting mad at the oblivious uploader.
one little caveat: at some point over the last few years the AR fix stopped affecting full-screened YT videos in Firefox in Windows 10. perhaps that is browser or OS dependent? ymmv.