2.5.2 Pointer Cancellation Level A

For functionality that can be operated using a single pointer, at least one of the following is true:

No Down-Event
The down-event of the pointer is not used to execute any part of the function;

Abort or Undo
Completion of the function is on the up-event, and a mechanism is available to abort the function before completion or to undo the function after completion;

Up Reversal
The up-event reverses any outcome of the preceding down-event;

Essential
Completing the function on the down-event is essential.

*Functions that emulate a keyboard or numeric keypad key press are considered essential.
*This requirement applies to web content that interprets pointer actions (i.e. this does not apply to actions that are required to operate the user agent or assistive technology).