The specification for HTML is in flux. While the current approved version is 2.0, the World Wide Web Consortium (W3C) has released a working draft for version 3.2. To quote from the draft:
HTML 3.2 is W3C's specification for HTML, developed in early `96 together with vendors including IBM, Microsoft, Netscape Communications Corporation, Novell, SoftQuad, Spyglass, and Sun Microsystems. HTML 3.2 adds widely deployed features such as tables, applets and text flow around images, while providing full backwards compatibility with the existing standard HTML 2.0.
The Web pages created by mtx2html have been checked for compatibility with HTML 2.0 and 3.2. You should limit the use of advanced (non 2.0) features if you want to keep your Web pages "generic" and available to the widest possible audience.