Have you ever read your DTD?
www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd
www.w3.org/TR/xhtml1/DTD/xhtml-lat1.ent
www.w3.org/TR/xhtml1/DTD/xhtml-symbol.ent
www.w3.org/TR/xhtml1/DTD/xhtml-special.ent