It's a common pattern to define all the primitives in your style guide as "global" (/ attempted global) custom properties on :root, e.g.

:root {
  --pale-blue: <etc>;
  --light-blue: <etc>;
  /* ... hundreds ... */
}

If I understand the highlight inheritance model correctly, such global custom properties would not be available on ::selection (etc), unless you modify the custom-property-defining rule to something like :root, :root::selection {}. In principle what the author would be doing is creating two separate mega-blobs of custom properties. This may or may not be easy to optimize depending on how the usage patterns will actually end up.

Authors can use @property w/ initial value to address it. Is that enough, or we do something else about this?

@delan @tabatkins