Search engine optimisation (SEO) and web accessibility techniques are quite different things with different aims, but sometimes they overlap: thus, improving the overall accessibility of a website can help the search engine indexing content and giving it a high rank, as well as make it accessible to a bigger audience.
What is web accessibility?
Web accessibility is a set of practices and techniques whose purpose is to make content in the Web accessible to the biggest audience possible, regardless of any kind of impairment or disability.
How does it affect Google Ranking?
The main reason why improving accessibility has a positive impact on SEO is that, when “navigating” a website, Google is essentially deaf and blind: its technical abilities are somehow limited and it can get information just if presented in the right way, like an assistive technology. Thus, exposing the content in this way give us the chance to be seen by Google, and so to be indexed and to be visible in a SERP for the relevant keywords, as well as by users using an assistive technology.
How can you increase your Google ranking with web accessibility?
Alternative content
The very first step is to provide alternative content in form of plain text to any non-text element of the web page, like images, videos or audio clips. This information will be consumed by a vast audience, including deaf users, users who are in a library and can’t listen to the audio (yes, an impairment can be caused by the current situation in which the user finds herself) and, of course, by the Google bot.
Anchors
Another bunch of accessibility techniques regards the anchors: for example, having a highly descriptive text, stating clearly where the anchor is pointing at (avoid “click here”) affects positively both the user experience and the ranking factor; checking that all the anchors are working (that is: no broken links) is another nice way to help your user and Google bot.
Structure
Structure plays an important role as well: HTML content should be coded using the appropriate tags, especially when it comes to headings. The headings of a web page are one of the most important piece of content, giving to the user (human and not) the general view over it and the relationship between the sections.
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Other accessibility improvements are more related to the user experience and the design, and they can affect “quality signals” like the time spent on a web page or the bounce rate. For example, a website with a very messy architecture and a confusing navigation is more likely to be early abandoned by the user. This information is used by Google to determine the quality of content: pages whose time spent on them is really little are low ranked, thus showing far away from the top results in a SERP.
At the same time, a usable website is more likely to be referenced, or backlinked, from other sources (other websites, blogs, etc): the more is referenced, the higher is ranked.
Finally, a special mention to mobile-friendliness: today is one of the most important ranking factors for Google.