How will your page look on different systems (OS/Browsers) ?
Specifying fonts or sizes => CSS
Never use non standard HTML Tags
Can common browsers view your page
Do you include metadata (TITLE tag and ALT attributes and others)
Try on several systems/plateforms
Safari, Firefox, Omniweb, Netscape Navigator, OPERA, Lynx ( text only ), MSIE.
Can search engines extract information from your page
Do you want them to ?