a “small class of input�? any gesture (which is by nature time-dependent) that requires a mouse by definition cannot be done with a keyboard. Similarly for a pointing pen (like signing on a screen). or using a joystick to fly a virtual airplane. I think that as long as the user clicks with the mouse then that can be reasonably easy and similarly be implemented with keyboard interaction. Clicks and movements (like in drag and drop) somewhat less easily and similarly.
Thank you for pointing out the ambiguity of the wording. The SC wording has been changed to:
2.1.1 Keyboard: All functionality of the content is operable through a keyboard interface without requiring specific timings for individual keystrokes, except where the underlying function requires input that depends on the path of the user's movement and not just the endpoints. (Level A)
Note: This exception relates to the underlying function, not the input technique. For example, if using handwriting to enter text, the input technique (handwriting) requires path dependent input but the underlying function (text input) does not.
Note: This does not forbid and should not discourage providing mouse input or other input methods in addition to keyboard operation.
In Understanding 2.1.1, we added: The phrase "except where the underlying functionality requires path dependent input" is included to separate those things that cannot reasonably be controlled from a keyboard.
Most actions carried out by a pointing device can also be done from the keyboard (for example, clicking, selecting, moving, sizing). However, there is a small class of input that is done with a pointing device that cannot be done from the keyboard in any known fashion without requiring an inordinate number of keystrokes. Free hand drawing, watercolor painting, and flying a helicopter through an obstacle course are all examples of functions that require path dependent input. Drawing straight lines, regular geometric shapes, re-sizing windows and dragging objects to a location (when the path to that location is not relevant) do not require path dependent input.