UPDATE (May 13: 9:30 am PDT, 17:30 GMT): We believe the problem is resolved. Please let us know in comments if you’re still seeing issues with the formatting of HTML messages posted AFTER 9:00 am PDT (17:00 GMT).
We’re aware of the email formatting problems affecting many groups and messages and our engineering team is working to resolve the issue. We’ll provide updates to this blog post as we have them. Sorry for the hassle.