(EXtensible HTML) A markup language for Web pages from the W3C. XHTML combines HTML and XML into a single format (HTML 4.0 and XML 1.0). Like XML, XHTML can be extended with proprietary tags. Also like XML, XHTML must be coded more rigorously than HTML. Over the years, HTML coders have become sloppy, because Web browser software was originally written to tolerate many variations in HTML coding. With XHTML, coders must conform to the rules. See HTML, XML and XHTML+Voice.