As you design your site and add content, you should be considering best practices for users who are visually impaired or who browse with screen readers. It is fortunate that much of the basic advice regarding accessibility can also help with search engine optimization. "Google is blind" is a phrase you hear a lot in introductory SEO circles.
"The next time someone stands up in a design meeting and claims that you don’t have any blind customers, ask them if they care about search engine placement. Then remind them that Google is a blind user who reads the entire Internet every month, and then reports what it sees to millions of its closest friends." (quote attributed to Mark Pilgrim)
Basic pointers about accessibility
- Use text wherever possible
Anything that is hidden inside of a graphic, flash embed, or multimedia document will be harder to access for screen readers and search engines. We usually recommend that you add introductory text that incorporate keywords about the content when you add extras such as PDF downloads or Flash widgets to make it easier for search engine indexing to occur. - If you use graphics, use the ALT tag
Don't forget to add ALT text ("alternative text" to describe the content of the image) to help contextualize for your screen reader users and search engine crawlers. - Use clean markup and cut unnecessary code
Keep it clean, it makes it easier for everyone. Avoid tables for layout where possible. If you use Javascript, make sure it degrades for users who have scripts disabled (ie. make sure the page still functions in a stripped-down form).
Further info
Accessibility for all users, even search engines
...The goal of accessibility is to make web content accessible to as many people as possible, including those who experience that content under technical, physical, or other constraints. It may be useful to think of search engines as users with substantial constraints: they can’t read text in images, can’t interpret JavaScript or applets, and can’t “view” many other kinds of multimedia content. These are the types of problems that accessibility is supposed to solve in the first place.
From "High Accessibility Is Effective Search Engine Optimization" by Andy Hagans
