Rocket Validator integrates the W3C Validator HTML checker into an automated web crawler.
HTML issues tagged as xmlns:fb.
Both xmlns:fb and xmlns:og attributes have been deprecated. If you’re using Facebook / Open Graph Protocol proprietary tags in you document, you can update the markup to the new syntax.
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.