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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.WEBKIT_FORCE_AT_FORCE_MOUSE_DOWN is a proprietary, WebKit-specific, static numeric property whose value is the minimum force necessary for a force click.
Because WEBKIT_FORCE_AT_FORCE_MOUSE_DOWN is a static property of MouseEvent, you always use it as MouseEvent.WEBKIT_FORCE_AT_FORCE_MOUSE_DOWN, rather than as a property of a MouseEvent instance.
Specifications
Not part of any specification. Apple has a description at the Mac Developer Library.