MouseEvent: webkitForce property
Non-standard: This feature is not standardized. We do not recommend using non-standard features in production, as they have limited browser support, and may change or be removed. However, they can be a suitable alternative in specific cases where no standard option exists.
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.