Without the -t option, tbook creates XHTML 1.1+MathML files. XHTML is the upcoming standard in the Web. But in order to read XHTML properly with a web browser, two conditions must be met:
But older browsers (I tested Netscape 6.2 and Internet Explorer 6) display it quite well (of course not the MathML parts).
XHTML pages that are served as ordinary HTML and that don't contain MathML may be displayed quite appealing.
Hickson's Trick: If you insist on proper MathML rendering, an interesting trick is to serve the same XHTML file as HTML for Internet Explorer and as XHTML for all others. Then both big browsers can view the MathML parts: Netscape/Mozilla via intrinsic features, and the IE via the free MathPlayer3 plugin. For this trick, have a look at http://software.hixie.ch/utilities/cgi/xhtml-for-ie/.