Accessibility
In most educational settings, accessibility (the ability for users with certain disabilities to access Moodle's functionality) is now a legal requirement. So, it is important to make sure that your system complies with the respective standards. An area has been dedicated to Moodle accessibility in Moodle Docs, which you can access at https://docs.moodle.org/en/Accessibility. It provides useful links to standards, guidelines, legislations, and also subject-related tools and resources.
Guaranteeing accessibility through Moodle themes
CSS is Moodle's representation layer that is independent from the content layer, which is represented in XHTML 1.0 Strict. Thus, accessibility can be achieved through the theme itself.
Once you have implemented your accessibility styles, either directly in the theme or through Custom CSS, as shown earlier on, Moodle provides links to three external sites, which check the current page for standard compliance. To activate these, go to Development | Debugging...