Some may prefer to use one mode of input, such as a mouse, keyboard, or keyboard-like interface, over others when interacting with web content. To ensure that people have the option to utilize the input mechanism that provides the greatest ease of use, WCAG recommends that web content not restrict users from switching between mechanisms, or using one instead of another, except in cases where it is deemed essential or crucial to preserving user settings or security of the content.
We often employ a number of input mechanisms at the same time when operating the web (think a mouse and keyboard on a desktop computer). However, some devices may have a primary input modality, like a touch screen on a phone or tablet. Even in these cases, it is important that users have the option to implement an alternative mechanism. For instance, those with certain motor limitations may have trouble manipulating a touch screen and instead relies on a paired mouse or keyboard to use their portable device.
As noted in the previous section, this rule does not apply in cases where it is essential that one input modality
An example where it would be "essential" to use a specific input mechanism would be a web application that teaches users touch-typing (any other Mavis Beacon fans out there?). Without a keyboard in such a situation, needless to say, the program would just be a waste of time.
WCAG has designated this criterion to be Level AAA, since it is not strictly needed for your page to be operable, though it will help ensure your page is more accessible. See our post on WCAG's levels of conformance for more information.