The reason it's not working

Hey aone! Sorry, I don't read greasyfork messages so I didn't see your July 27 message until now.

You said that you wanted to turn [the script](https://greasyfork.org/en/scripts/23329-disable-youtube-60-fps-force-30-fps) into a Safari extension.

I can see why your extension is not working. You're waiting until "document is ready" and "document.body exists". By that time, YouTube has already executed its own scripts that do the format checking.

This "format support-checker" code override/injection must run as soon as possible, before YouTube's own scripts have queried the format-checking functions. So it must run at "document-start", in Tampermonkey terms. At such an early injection time, often "document.body" won't even exist.

Anyway, I just updated to version 1.5 of the script today to change the injection success check method, because YouTube (or Tampermonkey) recently [broke something strange](https://greasyfork.org/en/forum/discussion/28835/x?locale=en). That's when I saw your message at GreasyFork! ;-P