The fact that HTML5 basically completely specified the parsing for any string of input, even "syntax error" cases, raises an interesting point: if these errors still result in some DOM and across all browsers that choose to implement the error handling (which has also been standardised) so they will have the same behaviour, are they really true "errors" anymore? We usually think of error cases (e.g. in a programming language) as ones which have no meaning or could cause implementation-defined/undefined behaviour, but these have been completely defined by the standard.

I don't see any good reason to use "</br>", but there's some other cases that could be useful, like not requiring spaces between quoted attributes (name1='value1'name2="value2"). I see a parallel with this and the evolution of natural languages: words and syntax that used to be incorrect gradually become accepted as part of the language and attain a normative meaning, because everyone still understands.