XHTML was an attempt at such strictness. It failed, though you can still use XML syntax for HTML if you choose to (serve it with MIME type application/xhtml+xml

XHTML was an attempt at such strictness. It failed, though you can still use XML syntax for HTML if you choose to (serve it with MIME type application/xhtml+xml instead of text/html). I leave you to research why it failed if you want; plenty has been written on the topic. In the mid-2000s, HTML syntax was finally codified. There is now no undefined behaviour: all inputs have a defined output, however surprising may be, because a lot of it was being relied upon.