In https://drafts.csswg.org/css-scoping-1/#shadow-names, it is specified that shadow names are inherited and can be used by descendant shadow roots.
However, in the case of @keyframes, no browser implements this. see https://css-names-in-the-shadow.glitch.me/anim.html, the 3rd row (inner element, outer keyframes), and despite appearing in the scoping spec, there is no mention of it in the css-animations spec.

Proposing to be explicit in the spec about current browser behavior, as changing all the implementations now might be too risky in terms of backwards compat? (If we changed all the implementations, what would probably happen is that things that didn't animate before due to an outer-scope @keyframes would now animate.