CSS counter-set is an additional property introduced in css-lists-3 to control counters by setting an existing counter to a specified value. This builds upon other implemented counter control properties, specifically counter-reset (which creates a new counter with a specified value) and counter-increment (which increments an existing counter by a specified amount).
This feature is needed for precise counter control, since otherwise it is not possible to set an existing counter to a value, other than figuring out how to increment or decrement it.
Docs: https://drafts.csswg.org/css-lists-3/#propdef-counter-set
Explainers: https://drafts.csswg.org/css-lists-3/#propdef-counter-set