[css-values-5] What does sibling-index()/sibling-count() return if there's no originating element?

[sesse]

sibling-index() (and its sibling, sibling-count()) does not seem to have defined behavior if there's no originating element, e.g. in a media query. What should the behavior here be? (Returning 0 would be fine by us, if you just want to pick something arbitrary.)

[Loirooriol]

I don't think we should defer to the ultimate originating element at all, see #9573