"HTML" in this context is meaningless.  Are you talking about XHTML, HTML4, HTML5 or what? ;)<p>It depends on the doctype and namespace.  In XHTML, an uppercase HR is technically meaningless, since it is case sensitive.  In HTML5, it is not case sensitive so hR should be valid, according to the spec.<p>Citation:
"In the HTML syntax, tag names, even those for foreign elements, may be written with any mix of lower- and uppercase letters that, when converted to all-lowercase, matches the element's tag name; tag names are case-insensitive."