Even if `<hr />` (or others, e.g., `<img />`) would be required XML processing would not work. HTML (prio to HTML5) was full of quirks (e.g., table handling, formatting elements, ...), which cannot be expressed by a DTD that is used from XML. As a result, the DOM as seen from an XML POV could always be different from the real DOM even if the source could be parsed.

HTML5 just standardized all these quirks, leading to a uniform parsing model instead of an even bigger x-browser mess.