I think we have a resolution to make new OM interfaces read-only somewhere, but I couldn't find it. #6819 / #7033 (comment) etc are pointers tho...

In any case, any strong reason to make https://drafts.csswg.org/css-view-transitions-2/#navgation-behavior-rule-interface writeable? If not, could we just make it read-only?

cc @nt1m @noamr @tabatkins