To write accessible HTML you need to take care of several things. Some are easy to implement if you know how, some take a bit more effort.

The most important thing, because you have an interaction and not only a one way to the reader, is a form. You have to declare a relation between the input field and its label, to get a better readability. Grouping is also important for better overview. Look at the competition from our examples.

Whenever you have a link or a button which is not clearly labeled, you can add a description and hide it through Style sheet. The screen reader will read it anyway.
Take a file download as an example. Provide the file's size and extension.

<span class="hidden">(pdf, 230kb)</span>

For text size, never use the FONT tag with a fixed SIZE attribute, instead use Style sheet and declare a relative value.

When adding images to your content use the ALT attribute. If you don't want or need one, leave it empty but still place it. Otherwise a screen reader reads the filename which is not that useful. For a longer description of your image use the LongDesc column from the image library. BKSImage renders the description in a separate web page.

Page layouts made of tables may also become a problem if not well handled.

This is just a small slice of web content accessibility to show that there are several things to have in mind when designing a website.

For further information about accessible HTML please read the W3C Accessibility Guidelines: http://www.w3.org/TR/WCAG10-HTML-TECHS/