validate-website 1.0.0 !
validate-website has reached 1.0, it's been a while I'm working on this gem and it helps me find many of validity errors, for example on Debian website and ruby-lang.
It's great when your work is useful and has been included on the Gemfile of ruby-lang project.
I still need to find a way to validate html5 pages without validator.nu service, but it's fine for now.
Edit: You can use vnu.jar
for html5, more info
here.