In my experience a lot of the "best practice" warnings are reductive.

A couple examples from this list are not requiring a type parameter on certain tags, or style tags being valid under html and body tags but not div tags. These types of things would take extra coding in a browser to check for certain singular tags not requiring things that all others do. Therefore it is simpler to leave these checks out and leave sloppy syntax checking as it does no harm at all to the rendered markup.

You may be right in some situations, but the nature of HTML having everything applied to all tags and then selectively reduced definitely helps make it a more resilient and accommodating language.