Even the MDN docs show "hr" examples without closing: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/hr
Though they prefer to show "p" with closing tags (they mention it's optional though, and auto-close when one of several other tags are found: https://developer.mozilla.org/en-US/docs/Web/HTML/Element/p).
HTML was designed to be written by hand as previous generations of typesetting languages. I think this is the main difference from XML, and this was the reason of a war in the 90's about making HTML a sub-set of XML: see XHTML (https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/XHTM...)
