Yep, Richard is absolutely right. With the built in composer, we get an additional benefit because with the built in composer, we are actually generating html codes for you without you having to fiddle around with any html codes.
Having said that, if you do eventually use the HTML block, it would probably lead to the same errors. For an instance, if you view the post at
https://stackideas.com/blog/how-to-upgrade-to-joomla37 , Sylvie actually used a HTML block to design the coupon code and this page doesn't validate with AMP because of that,
https://search.google.com/search-console/amp?id=C09qySEFC95qSnT_SUpkFw
This however, will be fixed on the next release as we would try to search for known problematic attributes to AMP and yes, Google is being a pain in the a$$ for it!