When designing your site, you should be aware that not every person will be physically able to use a mouse to browse it, or even see what's on your pages. There are special browsers for disabled people, which try to convert webpages into spoken text or other media like braille pages. Other browsers allow to navigate webpages by using only a few keys. To improve the readability of your site for those people, there are a few things you should be aware of:

Always provide a short description in the "alt" attribute of IMG and AREA tags. When creating such tags, TagWriter will always ask for a description to remind you of this. Of course you should not provide a description for irrelevant images like pure decorational artefacts.
A similar thing counts for tables: you should use the "summary" attribute of the TABLE tag to provide information about a table's contents, unless the table does not contain any relevant data or the data has its own description.
Split up large forms into fieldsets with the FIELDSET element, and provide a LEGEND element in each of them. This also makes the form more surveyable in visual browsers.
Connect INPUT text fields and checkboxes and TEXTAREA objects with their textual label in the HTML file by using the LABEL element. The safest way is to assign an "id" attribute to each input element, and then put the describing text inside a LABEL element with a for-attribute which points to the object's ID. If the input element follows directly after the label, you can also simply put both inside a LABEL element, like in this example:
<LABEL>Enter your name: <INPUT type="text" name="Name"></LABEL>
By providing an accesskey attribute for links, image map areas and form input elements, the user can access these objects by pressing a single key. Of course you need to mention somewhere which keys correspond to which objects.
Using style sheets instead of "classic" page layout techniques in order to improve the distinction between document structure and presentation, does not only ease your work, but also allows easier interpreting of your documents.
As said in the general tips section: when using frames, make sure to provide a NOFRAMES part in a frameset page with links to the most important pages in your site.