This is not quite true. There are a number of mutual incompatibilities between the XML and HTML syntaxes at both parse and run time.

At parse time, it’s mostly in the direction of XML syntax making things possible (e.g. nesting paragraphs or links, which the HTML parser prevents), but also in the other direction (e.g. <noscript> has no effect in XML syntax since it’s essentially an HTML parser instruction); you’ve also got case sensitivity which matters for SVG; and there’s the matter of the contents of <script> and <style> elements and their handling of <>&, where the best but still imperfect solution is a crazy mix of XML comments, JavaScript/CSS comments and XML CDATA markers. (See https://www.w3.org/TR/html-polyglot/ for more details of all this kind of stuff.)

At run time, behaviour changes in such a way that it will break some JavaScript libraries, due to differences like .tagName being lowercase instead of uppercase, and .innerHTML requiring and producing XML syntax.