But the SGML heritage is what makes this valid, and it is common in other SGML doctypes. (e.g. if you download OFX data from your bank, the closing tags will b

But the SGML heritage is what makes this valid, and it is common in other SGML doctypes. (e.g. if you download OFX data from your bank, the closing tags will be omitted from certain elements.) Both the optional closing tags for P and the optional opening tags for stuff like HTML, BODY, TBODY, etc. are present in the HTML 4.0 DTD too. (A TR element cannot go in a TABLE element, there is an implicit TBODY.) And SGML, HTML, and optional tags go back to before XML existed.