Accessibility and SEO overlap at a fundamental level - search engines, because of the way they are built and operate, face many of the same obstacles as users with disabilities. For example, a search engine cannot "see" or understand certain images or video files, nor interpret vision-based text like ASCII art, nor "hear" certain audio files.

Accessibility features make it easier for search engines to crawl and interpret websites. For example, sites with clear, descriptive headings - the same kinds of headings that also make navigation and comprehension easier for people for disabilities - are better optimized for search engines to do their work.

Because of this, Google rewards accessibility when ranking websites. In fact, their Webmaster Guidelines - which lay out the best practices that help Google to find, index, and rank your site - read very much like accessibility guidelines, and often correlate directly with the W3C's Web Content Accessibility Guidelines (WCAG).

So by adding features like meta descriptions, HTML sitemaps, and breadcrumbs, you are simultaneously improving the on-page experience, making your content more accessible to users with disabilities, and facilitating the work of search engine bots who are busy crawling, indexing your site, and assessing link equity between pages - all of which affect your SEO ranking.

This means that the converse also stands, that if you fail to include the right accessibility features, say alternative text for images, you are creating obstacles not just for users with disabilities, but also hurting your image SEO.