If you plan a website you should know that by creating W3C compliant, usable and accessible websites you increase your SEO score, but most importantly you make a valuable contribution to improving the Web and creating equal opportunities for all the users.
Accessibility is expensive, some say. And it’s true for large corporations that need to optimize large sites (although CSS makes even that easy). But small websites don’t need to invest a fortune in developing accessible websites. They just need to respect the standards, and if they cannot employ people with disabilities to test the sites, aDesigner from IBM is enough. I encourage you to download it and use it to test all your website.
Now how does accessibility improve your SEO score? Well, defining the proper ALT attributes for your images is had no SEO value in the past, but only a value for the blind, who couldn’t see the images and needed text instead. Now, with the growth of image search, the ALT attributes bear SEO value and they help you rank in the image search results for keywords they contain.
The universal search is the next logic SE development. So by creating text captions for video and sound, you do make it easier for the users with disabilities to understand the content of your pages, but you also help the search engines properly index those files.
I hope these highlights made it clear for you why so many SEOs encourage you to correct HTML errors and to create accessible sites.