Thanks for your contribution! This thread is meant to collect all methods to run YouTube and its videos under Windows XP in these days. I already wrote about SMPlayer here:
https://msfn.org/board/topic/176299-latest-version-of-software-running-on-xp/?do=findComment&comment=1258461
https://msfn.org/board/topic/176299-latest-version-of-software-running-on-xp/?do=findComment&comment=1258479
For watching YouTube videos, I personally prefer PotPlayer and maroc's release of MPC, now in the version 2.2.1. Inside of UXP browsers on old, weak computers, I would use the extension uTube which is absolutely great. SMPlayer is unfortunately no longer supported but it is also a good one as long as it still works under Windows XP, of course. My experiment in a previous post was only meant to check if YouTube can be used natively inside UXP browsers on old, weak computers in these days.