Use features that enable activation of page elements via a variety of input devices.
Device-independent access means users can interact with the user agent or document with a preferred input (or output) device -- mouse, keyboard, voice, head wand, or other. If a form control can only be activated with a mouse or other pointing device, someone who is using the page without sight, with voice input, or with a keyboard or who is using some other non-pointing input device will not be able to use the form.
Note. Providing text equivalents for image maps or images used as links makes it possible for users to interact with them without a pointing device.
Checkpoints:
9.1 Provide client-side image maps instead of server-side image maps except where the regions cannot be defined with an available geometric shape.
9.2 Ensure that any element that has its own interface can be operated in a device-independent manner.
9.3 For scripts, specify logical event handlers rather than device-dependent event handlers.
9.4 Create a logical tab order through links, form controls, and objects.
-For example, in HTML, specify tab order via the "tabindex" attribute or ensure a logical page design.
9.5 Provide keyboard shortcuts to important links (including those in client-side image maps), form controls, and groups of form controls.
-For example, in HTML, specify shortcuts via the "accesskey" attribute.
As you can see this would be a very important guideline to follow so that users who use other methods of interaction besides a keyboard or mouse to navigate throught a web page or gain information from a site. These seems like it would be a pretty difficult task, atleast at my level of understanding. Maybe that is why some websites do not follow this guideline. To get all the details about this guideline visit this link.