DebugBear now has a Validation tab which shows errors and warnings generated by the W3C HTML validator.
Most of these errors aren't very helpful. The HTML might not be valid, but as long as all browsers handle it fine that's not a problem. And sometimes the validator doesn't know about a recently added feature and will complain about it.
So DebugBear doesn't list common validation errors by default, and currently no email or Slack alerts are sent if there's a regression.
However, there are many potential problems the validator can identify:
duplicate attributes (e.g. two style attributes on the same element)
Stray start and end tags
Invalid inline CSS, like style="background: [object Object]"