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https://drafts.csswg.org/cssom/#the-getstyleutils-interface [[ [NoInterfaceObject] interface GetStyleUtils { [SameObject] readonly attribute CSSStyleDeclaration cascadedStyle; [SameObject] readonly attribute CSSStyleDeclaration defaultStyle; [SameObject] readonly attribute CSSStyleDeclaration rawComputedStyle; [SameObject] readonly attribute CSSStyleDeclaration usedStyle; }; ]] It appears these are not going to be implemented. Drop them from the spec.
https://github.com/w3c/csswg-drafts/commit/87f3670b7622df38b4ddcd6fa7675c0c015cc5b3