There may not be - I don't know Blogger and how to control it.
I confess that I was surprised that blogger was generating an HTML5 page. The errors on the message board page do all seem to come from the Blogger side of things, rather than Nabble. Most browsers are very forgiving of errors in code, to such an extent that many web designers disregard the W3C guidance. Maybe mobile browsers are different.
I've not read up on the topic, but as a guy who normally codes his own sites rather than rely on CMS, blogging and other services to generate my sites, (Nabble is the exception!) I'm a bit ignorant on that. Sorry I can't help more.
Volunteer Helper - but recommending that users move off the platform!
Once the admin for GregHelp now deleted.