Chromium-based Slimjet Browser with Continued Support for XP and Vista

It sounds impossible to make this browser secure. They're allowing Java and NPAPI plugins as an explicit compatibility goal.

Granted, security isn't a stated goal here. XP users unfortunately can't expect that anymore.

Are there any modern HTML5-compliant browsers that support XP anymore?

What's privacy in the absence of security? Last I checked, there is no ASLR support for Windows XP. You really do not want to run something with Chromes attack surface on it.