Tell them Haacked sent ya (not that I know those people).
As to the question whether this is XHTML, HTML, XML, SGML compliant? Who really gives a flying fuck? I mean c’mon people, get a freakin’ life! The real question is why choose a serif font over a sans-serif? ;)
For you real geeks: 0x3c 0x2F 0x62 0x75 0x73 0x68 0x3E
UPDATE: Oh, and this shirt is an XHTML compliant end tag (unlike the protest sign) assuming it is preceded by a start tag. According to Section 4.2 of the W3C recommendation, Element and attribute names must all be in lowercase.