I'm talking about the actual syntax outlined in the spec.
What syntax 'rules'?
You SHOULD use "well formed", valid, correctly closed and nested tags in ALL HTML coding.
You can use all lower case tag and attribute names in ANY HTML coding, and when all is said and done, the only thing that really separates HTML from XHTML is the DTD.
The only reason that XHTML
required the above, was for XSL/XSLT
Same with HTML 5, if you want to use HTML 4.01 or HTML 3.2 for your code NO ONE can tell you that it is 'wrong' and you MUST use HTML5.
The HTML specifications are NEVER 'superceded', never made invalid or redundant they simply remain in force and usable long after the latest and greatest becomes "recommended".
If you do not need the additional features of HTML5 and use HTML 4.01 for evermore that's fine, the "markup police" are not going to round you up and incarcerate in some rodent infested dungeon because you did not use HTML5