A quick note right off the bat: you probably don't need to worry about this rule unless you're developing or scripting your own user interface components for your site, or you're just very interested in WCAG guidelines.
For any user interface component—including links, form elements, and components generated by scripts—it is crucial that the following be true:
This will help ensure that such components appear and function in the manner that they are intended to, especially for users who rely on assistive technologies, such as screen readers, speech recognition software, and screen magnifiers. For instance, users with certain disabilities may not be able to visually ascertain whether focus has shifted to an interface control or if a checkbox or radio button has been selected. Accordingly, it is important that any assistive technology they happen to be using is compatible with your site's interface components.
Right, but why did I say, "you probably don't need to worry"? Standard HTML controls will already meet the requirements laid out here. It's primarily if you are developing or scripting your own user interface components that you need to worry about such compatibility issues.