WCAG 2.0 Understanding Page

Intent

Users may employ a variety of input mechanisms when interacting with web content. These may be a combination of mechanisms such as a keyboard or keyboard-like interfaces and pointer devices like a mouse, stylus or touchscreen.

Even though a device may have a primary or preferred input mechanism, the user may choose to apply alternative mechanisms. For example, a primary mechanism is the touchscreen on mobile phones and tablets, while a paired mouse or external keyboard may be an alternative.

Dynamically adding, removing or switching a particular input mechanism should be doable at any point by a user if certain tasks and interactions are easier to accomplish that way. Content must therefore not limit the user's interaction to any particular input mechanism unless the restriction is essential, or is required to ensure the security of the content or to respect user settings.

Note: A touch-typing web application, which teaches users how to touch-type on a keyboard and/or measures their proficiency and speed, would be an example of an essential limitation to a particular input mechanism.

Benefits

Users can interact with web content with whichever input mechanism is preferred and available to them.
Users may switch between input mechanisms when they desire or the circumstances require it.
Users are allowed to add and remove input mechanisms at any point, where supported by the operating system.

Examples

A user with mobility impairment pairs a mouse and keyboard to her mobile phone with a touchscreen. The phone can thereafter be operated by those input devices and the content does not accept the touchscreen as the only input mechanism.
On a touch-enabled laptop with coarse precision, people who have difficulty activating a small target because of hand tremors, limited dexterity or other reasons are still able to interact with content using their keyboard and trackpad.
A user starts interacting with a page using a desktop keyboard, and then attaches a secondary touch-enabled monitor. Content can be operated using this newly added input mechanism and does not assume that the keyboard, the first input mechanism it detected, is the only one in use.
A speech input user navigates content using voice commands which translate to simulated keyboard commands. When talking with a colleague, however, the user switches speech recognition off and uses the mouse instead.

Techniques

Sufficient

Only using high-level, input-agnostic event handlers (focus, blur, click) in Javascript (operating systems/UAs fire these events for all input mechanisms).
Registering event handlers for keyboard/keyboard-like and pointer inputs simultaneously in Javascript (see Example 1 in Pointer Events Level 2)

Failure

Registering either touch event handlers or keyboard/mouse event handlers based on a feature check/presence of a touchscreen in Javascript.
Using CSS Media Queries Level 4 Interaction Media Features to determine what the UA deems to be the "primary" input mechanism and assuming no other input mechanisms are present/may be added/may be used.