Registered to add some additional info on this issue. Looks like YouTube pushed breaking changes today that stop OBS from setting innerHTML (and therefore injecting CSS). Here is a snippet of the logs that pop when this happens:
And in text for copy ability:
17:33:05.667: [obs-browser: 'Chat'] Error: This document requires 'TrustedHTML' assignment. (:1)
17:33:05.668: [obs-browser: 'Chat'] Error: Uncaught TypeError: Failed to set the 'innerHTML' property on 'Element': This document requires 'TrustedHTML' assignment.
17:34:34.329: [obs-browser: 'Chat'] Error: This document requires 'TrustedHTML' assignment. (:1)
17:34:34.330: [obs-browser: 'Chat'] Error: Uncaught TypeError: Failed to set the 'innerHTML' property on 'Element': This document requires 'TrustedHTML' assignment.
This sort of issue seems have a number of workarounds on a cursory inspection, but I don't know what might be best fit for OBS.