As I scanned of the first 10 hits at https://www.google.com/search?q=html5+for+windows+xp it was clear to me you have prior answers to read up on.
But ANY DRM will not be fixed this way.
I know my OS is old. But it works fine on my old computer. Recently I found that lots of sites with video, for example Youtube live stream, tend to use html5, my Mozilla refuses to play it, I wrote them and they said I have to install html5 support on WinXP. But how to install?? Someone advised Primetime html5 content decryption module. But it's not supported by Adobe anymore. So is there other way to make my Mozilla on Windows XP play html5 videos?