Just as important as taking advantage of the HTML features provided specifically for accessibility is taking care to design your pages in a manner that's as accommodating as possible for users who are in need of assistance. Most of these techniques are relevant to all users, not just those using alternative browsers or assistive technologies.
A common pitfall designers fall into is using color to confer meaning to users. For example, they print an error on the page and change the font color to red to indicate that something went wrong. Unfortunately, visually impaired users won't be able to distinguish your error message from the rest of the text on the page without reading it. Needless to say, putting two elements on the page that are the same except for color (such as using colors to indicate the status of something) is not accessible. You can add borders to elements that need to stand out, or you can label them with text. For example, you might display an error message this way:
<p >ERROR: You must enter your full name.</p>
When you specify fonts on your pages, you can cause accessibility problems if you're not careful. In some cases, font specification doesn't matter at all because the user accesses your site with a screen reader or alternative browser that completely ignores your font settings. However, users who simply see poorly can have an unpleasant experience if you set your fonts to an absolute sizeparticularly if you choose a small size. If a user has set his browser's default font to be larger than normal, and your pages are hard coded to use 9-point text, that user will probably dump your site altogether.
In many cases, it makes sense to leave the default font specification alone for most of the text on your site. That way, users can set their fonts as they choose, and you won't interfere with their personal preferences. If you do modify the fonts on the page, make sure that the fonts scale with the user's settings so that they can see the text at a size that is comfortable for them.
Take Advantage of All HTML Tags
It's easy to fall into the trap of using <i> or <b> instead of more specific tags when you need to add emphasis to something. For example, let's say you're citing a passage from a book. When you enter the book title, you could indicate to your users that it's a proper title by putting it inside the <i> tag, or you could use the <cite> tag. There are plenty of other underutilized tags as well, all of which provide some semantic meaning in addition to the text formatting they're associated with.
Even in cases in which you really just want to emphasize text, it's preferable to use <em> and <strong> over <i> and <b>. These tags provide a lot more meaning than the basic text formatting tags that are often used. Not all alternative browsers will take advantage of any or all of these tags, but conveying as much meaning as possible through your choice of tags won't hurt accessibility for sure, and will help some now and could help more in the future. There's no downside to taking this approach, either.
Frames and Linked Windows
Frames are, to put it bluntly, a barrier to accessibility. There are some workarounds available, but the bottom line is that if you're really concerned about accessibility, you should probably avoid frames. Using linked windows and pop-up windows can also be a huge hassle from an accessibility perspective.
If you opt to use frames, you should include titles for all of your frames, not just the document containing the frameset. Using regular browsers, the titles of these documents are suppressed. That's not necessarily the case with alternative browsers. Some will provide links to the individual frames when you pull up the frameset, and having titles makes it easier for users to distinguish between them.
Forms present another thorny accessibility issue. Nearly all web applications are based on forms, and failure to make them accessible can cost you users. For example, large online stores have a serious financial interest in focusing on form accessibility. How many sales would Amazon or eBay lose if their sites weren't accessible? Some work on making sure the forms that enable you to purchase items are accessible can really pay off.
One key thing to remember is that disabled users often navigate using only the keyboard. As I mentioned when talking about links, assigning sensible tabindex values to your form fields can really increase both the usability and accessibility of your forms. The other advanced form tags, such as fieldset, optgroup, and label, can be beneficial in terms of usability as well.