There was a time where HTML moved towards a more formalized XML-valid definition named XHTML. Ultimately, web/browser backwards compatibility and messy and forgiving nature lead to us giving up on that and now we have the HTML living standard with rules, but browsers (not sure to what degree it's standardized or not) are very forgiving in their interpretation.

While HTML, prior to HTML5, was defined as an application of Standard Generalized Markup Language (SGML), a flexible markup language framework, XHTML is an application of XML, a more restrictive subset of SGML. XHTML documents are well-formed and may therefore be parsed using standard XML parsers, unlike HTML, which requires a lenient, HTML-specific parser.[1]

XHTML 1.0 became a World Wide Web Consortium (W3C) recommendation on 26 January 2000. XHTML 1.1 became a W3C recommendation on 31 May 2001. XHTML is now referred to as "the XML syntax for HTML"[2][3] and being developed as an XML adaptation of the HTML living standard.[4][5]