This would allow actually resetting all properties to what they were before some arbitrary CSS was applied, e.g. in a subtree:

/* bunch of CSS here */

#vendor-controlled-section {
  &, *, *::before, *::after {
    all: unset; /* all `div`s in this section will be inline! 😱 */
  }
}

A couple ideas for the syntax:

1. Make unset a function that accepts which value to use for non-inherited properties; unset would default to unset(initial) but the above would use unset(revert). Maybe there's even a use case for unset(revert-layer)? 🤔
2. Add a new value named unset-revert.
3. Add a new value named reset. Would that be confusing / unclear what the difference is with unset?
4. Add two new values named revert-or-inherit and initial-or-inherit, and make unset an alias for the latter.