HTML Checking for Large Sites
Rocket Validator automatically checks your pages on the W3C Validator.
HTML issues tagged as FBML.
FBML - Facebook Markup Language was used to embed Facebook-specific tags on web sites, was retired in 2011 but this still appears on <html> tags on some sites. You should consider removing this from your site and use alternative ways to do that.
When was the last time you validated your whole site?
Keep your sites healthy checking for A11Y/HTML issues on an automated schedule.