https://drafts.csswg.org/css-view-transitions-2/#cssom

If you look at https://drafts.csswg.org/cssom-1/#the-cssrule-interface , you'll see that the numbers are purely historical. In fact CSS @scope and @starting-style both don't return any number.

@khushalsagar @noamr @vmpstr @bokand I assume this was just an oversight, but my suggestion would be to delete the partial interface CSSRule { bit.