Pass W3C 'unicorn' validator

Pass the W3C unicorn validator

Validator resides here.
I've already fixed index.html in 5bf63e71525d93f7e37e84a98e3aa70c403ccf09, the modifications should be reported in all other pages and in the template.
To test the page, spin up a temp web server serving the pages and open it to the Internet. Don't use local files.