I've used a lot and finally found that none of them is really compatible to xp and html5 at the same time anymore.
I preferred using opera 9 for the high speed, the low memory consumption (I've disabled JS, if that makes a big difference). I also love the functions (mouse gestures, site specific preferences, direkt drag'n'drop of links and images to the download-sidebar for dl). And of course the look after I've installed Lix skin!
Then I am using Opera 12.17 (last version based on opera#s own engine "presto" instead of the same engine as google chrome "webkit"). In the beginnin g I've used it to be compatible to sites using newer technologies which opera9 no longer can handle. Then I've found ExtendTube, which drastically increases the usability of youtube. And now I'm only using it to watch youtube videos without ads anymore.
Meanwhile for compatibility I'm using firefox and sometimes google chrome portable.
about the issues I've talked in the beginning:
Opera 9: no html5-support at all.
Opera 12: no html5-h.264-support.
FireFox (34+?): delivered h.264-decoders don't run at xp - and they diabled the possibility to use an own h.264-decoder (direct-show filter).
Chrome: working h.264-decoders but buggy displaying/drawing functions on xp (interfering with other windows; you bring another window in front of chrome and parts of chrome "flicker through").
So my conclusion: there is simply no solution fullfilling all needs for xp.
I'd suggest chrome if you want tu watch videos.
I'd suggest firefox for compatibility and privacy.
And I'd suggest the outdated opera 9 for speed and performance (and a customizability, that has afaik no more been reached in browser history).