Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
MouseEvent.webkitForce is a proprietary, WebKit-specific numeric property whose value represents the amount of pressure that is being applied on the touchpad or touchscreen.
Specifications
Not part of any specification. Apple has a description at the Mac Developer Library.