August FYI-
It's a tool that reports errors in "syntax or formats" as compared to THE W3 STANDARDS. It's a valid way of seeing how compliant your site's code is - and the 365 editor clearly produces some non-standard code. The search engines care more about content than code though, so the validator should be used primarily to correct errors that impact the user experience -which is important too - rather then scare people about their effect on seo.