I strive to build websites using best practices and the latest industry standards so that they're universally accessible. I've even won an award for it. That means paying attention to all the minutae of writing valid, semantic markup and separating your content from its presentation.
Think about how a visually impaired person would browse your website. Most of them use assistive devices like screen readers. These screen readers scan the markup of your site and attempt to read its content to the user. Alternative navigation techniques are often employed, allowing visually impaired users to "scan" a web page by tabbing through headings and labeled links. I engineer websites so your content can be easily interpreted by these assistive devices.
Paying attention to accessibility also helps your SEO. After all, Google's web crawler is said to be the most active blind user on the Internet. And if Google's web crawler can easily index your content you'll probably end up higher in search engine result pages.