Page "Markup language" Paragraph 28

One of the most noticeable differences between HTML and XHTML is the rule that all tags must be closed: empty HTML tags such as < code >< nowiki ></ nowiki ></ code > must either be closed with a regular end-tag, or replaced by a special form: < code >< nowiki > < nowiki >/></ nowiki ></ code > ( the space before the '< code >< nowiki >/</ nowiki ></ code >' on the end tag is optional, but frequently used because it enables some pre-XML Web browsers, and SGML parsers, to accept the tag ).

Another is that all attribute values in tags must be quoted.

Finally, all tag and attribute names must be lowercase in order to be valid ; HTML, on the other hand, was case-insensitive.