Element: webkitmouseforcedown event

After a mousedown event has been fired at the element, if and when sufficient pressure has been applied to the mouse or trackpad button to qualify as a "force click," Safari begins sending 
webkitmouseforcedown events to the element.
webkitmouseforcedown is a proprietary, WebKit-specific event. It is part of the Force Touch events feature.
Syntax
Use the event name in addEventListener():
addEventListener("webkitmouseforcedown", (event) => { })
Alternatively, assign to the event handler property of the interface:
onwebkitmouseforcedown = (event) => { }
Event type
A MouseEvent. Inherits from UIEvent and Event.
Specifications
Not part of any specification. Apple has a description at the Mac Developer Library.
Browser compatibility
See also
Learn: Introduction to events
webkitmouseforcewillbegin
webkitmouseforceup
webkitmouseforcechanged