XHTML is a stricter and cleaner version of HTML. XHTML 1.0 became a W3C Recommendation January 26, 2000. W3C defines XHTML as the latest version of HTML. XHTML will gradually replace HTML. Since XML is case-sensitive, XHTML element and attribute names must be written in lowercase. You can no longer get away with what many people used to do to improve readability of code — typing the element and attribute names in uppercase and the values in lowercase.