The gist is that:
1. Highlight Styles now inherit _regular_ CSS properties through the pseudo highlight chain, rather than the element chain.
This means you can set `main::selection` to set highlight styles for `main` + all its descendants. Before it was only `main` that would get the styles.
2. Excluded from 1 are _custom_ Properties. Those inherit from the originating element.
That way you can access custom props declared on `:root` from within `::selection { … }` but also that you can change a customprop on an element and the `::selection` will use the new value.