you say “unless the task requires analog time dep. input�. But the task is defined by the designer and is supported by the GUI. By definition, if the task requires some mouse based input that is analog time dependent, then the page succeeds 211.
Proposed Change:
I think you should focus on the concept of “goal� rather than task. A goal is a desired state of things that a user wants to achieve, and tasks are the observable actions to achieve the goal. Thus you might want to say that the goal should be achievable through 1 or more tasks that do not require analog time dependent gestures. Except when for that goal there is no other way (like piloting a plane!).
We agree with the concept of your comments but chose to address it with different wording as follows.
Changed SC by adding "underlying" to read
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.