The Accessibility Awareness Key is for sites passing our automated testing. The spider can check for some basic accessibility and usability issues - which will mean a site is readable to people of a lower reading age or for whom English is a second language.
You should style your pages so that users can enlarge fonts or make them smaller by using no fixed font sizes or column sizes. You should also separate structure from presentation. Doing all of these things means the site meets basic readability requirements. Users can, if you meet these criteria, to some extent, create their own settings to make your pages easier to read. You should also try to write as clearly and simply as possible and avoid using deprecated tags.
Automated accessibility testing can only look for specific things in the code of a web page that it has been designed to see. We can find, using an automated check, validation issues, serious errors in the code of a web page, deprecated elements which might cause problems for some users and problems which can be mathematically tested such as contrast between foreground and background colours and the difficulty of the text on a page using a formula to calculate readability. We can also check for code which indicates the use of scripts which may cause problems for some users and the use of a <noscript> tag.
Many of these elements should be used as warnings only in order to look further at the content and design of a web page. For example, while our spider may find that your site has provided alternate text for every image, it may be that this alternate text is not useful to people using a screen reader or that a description such as "spacer" has been assigned to decorative elements rather than null alt text. It is also possible that a site failing an automated check will pass manual checks because while it doesn't meet the letter of the law it is accessible in the real world. For example, a site using a DTD of XHTML 1.0 Strict may fail a validation check but changing the DTD to XHTML 1.0 Transitional would mean it would pass.
As a result, these tests are not infallible and site owners should always study the reports they receive about their site to locate problem areas and to find ways to improve the accessibility of their site. If your site does not pass the automated testing, it is always advisable to submit your site for manual testing which is far more nuanced and involves detailed checking of your code as well as testing of your site using a screen reader and without a mouse.
Make certain foreground and background colours have sufficient contrast. The spider can check for the relationship between text and background but not for colour contrast levels on images. Sufficient contrast is important to help people read the text on your site more easily - particularly for users with low vision, colour blindness or just tired eyes.
Read about how Colour Contrast criterion scoring works.
Do not use fixed font sizes so that users can make the size of the font bigger or smaller on their screen depending on their own personal preferences.
Read about how Font Size criterion scoring works.
Avoid using flickering or moving content. It can distract users and can even cause seizures in people with epilepsy. The spider can check for the <blink> and <marquee> tags.
Read about how No Flicker criterion scoring works.
Use language which is clear and simple as possible for your subject matter. Ideally you should have a Flesch-Kinkaid score between six and ten to make your site readable to the average user. The spider can test on the Flesh-Kinkaid scale but should you fail this test a manual check can ensure you meet the needs of your target audience.
Read about how Read Index criterion scoring works.
Separate structure from presentation so users can create their own user-defined style sheets to make pages easier to read for themselves.
Read about how Structure criterion scoring works.
Your site should work without JavaScript so users can turn off scripts if they choose to make everything run more quickly - or in case they are using older software or an older computer.
Read about how Without JavaScript criterion scoring works.