from w3ctag/design-reviews#584 (comment)

@hober and I discussed this today in our VF2F. We noticed that HighlightsRegister is a setlike, but throws when a Highlight object with an existing registered name, which indicates that perhaps maplike would have been more appropriate? It's not ideal developer experience if developers need to iterate the entire data structure before registering their object, to check if an existing one of the same name exists. Maplike would also provide an easy way to get all ranges under a given name.