[TEST] html5validator

[tdelmas]

https://validator.w3.org/nu/about.html https://github.com/svenkreiss/html5validator Fix:

• https://github.com/letsencrypt/website/pull/928
• https://github.com/letsencrypt/website/pull/927
• https://github.com/letsencrypt/website/pull/926
• https://github.com/letsencrypt/website/pull/925
• https://github.com/letsencrypt/website/pull/924
• https://github.com/letsencrypt/website/pull/940
• https://github.com/letsencrypt/website/pull/944 TODO;

• 31/#32

• Last invalid items

NB: htmlhint is still useful, for example to detect "ad" classes and to force a coding style.