The spec seems to imply that ::selection and ::inactive-selection are mutually exclusive, when actually ::selection applies to inactive selections currently. The spec should be updated to make their interaction compatible with this behavior.

Possibilities:

• ::selection and ::inactive-selection apply to the same highlight pseudo-element, possibly with ::inactive-selection at a higher specificity
• ::selection and ::inactive-selection are separate highlight pseudo-elements that both apply to the range of an inactive selection, with ::inactive-selection at a higher level than ::selection (in the same way ::spelling-error and ::grammar-error layer when applied to the same text)

Interested in feedback from any implementers, on what makes sense implementation-wise.