The Pareto principle (also known as the 80/20 rule) states that, for many events, 80% of the effects come from 20% of the causes. This is true for accessibility where a small number of issues cause the vast majority of problems. But what are these issues? That is a subjective question, but here are my top 5:
Poorly described images
By now you probably all know that images should have associated alt attributes, which describe them to visually impaired users and search engines. However, a related problem is the content of these alt attributes.
Many people have realized the benefit of alt attributes for search engine placement and so stuff them with keywords making them far too long.
All content images should have an alt attribute that clearly describes what is being shown in a concise manner.
Badly labelled links
It is not just images that are labelled badly. There are also problems with links. The text contained within a link should describe that link without context. This is because screen readers have the ability to read all links on a page as a single list. Users can then quickly navigate without listening to the entire page. However this is problematic because a link entitled ‘click here’ does not explain where it leads. A better link would read ‘click here for latest news’ or simply ‘latest news’. Where a longer description is required a title attribute can be added.
Descriptive links also help sighted users to quickly scan for the next page to visit.
No alternatives to media
It is not just images that need describing. When using video, audio or any form of media that requires a plugin (that some users may not have) it is necessary to provide an alternative version. This alternative should either be in the form of a transcript (in the case of audio) or captions (in the case of video or other media where visuals and audio are synced).
At first glance this seems a massive undertaking. However, there are a number of services like castingwords.com who provide transcription at a very reasonable rate. There are also tools like overstream.net, that help create captions.
User controlled text
The final accessibility mistake I see regularly is text that cannot be resized. By default all major browsers allow users to set the size of text on a webpage. This is needed because website owners cannot predict users visual requirements. Most people with visual problems need to be able to increase font sizes. However, there are some visual impairments that require smaller text to fit within a limited field of view.
Although browsers provide this functionality by default, many web designers disable it. To be brutally honest there is no good reason for this beyond laziness. By fixing the font size the designer reduces the burden of testing but it provide no other tangible benefit. In short, ensure the fonts on your web site are scalable.
By addressing these five problems you will dramatically improve the accessibility of your website. None of these issues are particularly hard to overcome and the financial investment is minimal. However, by doing so you will increase the amount of traffic to your site and the number of visitors able to successfully navigate it.