[web-animations-1] What if the effect target is a nested pseudo-element?

From https://drafts.csswg.org/web-animations-1/#keyframe-effects,

In order for the effect target to have a well-defined lifecycle, it is represented as an Element known as the target element and a pseudo-element selector knows as the target pseudo-selector. If the effect target is an Element, the target element is that element and the target pseudo-selector is null. If the effect target is a pseudo-element, the target element is its originating element and the target pseudo-selector is as required to specify that particular pseudo-element.

This seems to assume that pseudo-elements are always originated by an element. But nested pseudo-elements are originated by another pseudo-element.

So what happens if the effect target is a nested pseudo-element like ::before::marker?

I see 3 reasonable possibilities:

• The target element is the originating element of the ::before, and the target pseudo-selector is ::before::marker.
  That is, enforce the target element to be an Element, never a CSSPseudoElement, and let the target pseudo-selector be multiple pseudo-element selectors.
• The target element is the CSSPseudoElement represeinting the ::before, and the target pseudo-selector is ::marker
  That is, enforce the target pseudo-selector to be at most a single pseudo-element selector, and let the target element be either an Element or a CSSPseudoElement. Just forbid the case of the target element being a CSSPseudoElementand the target pseudo-selector being null.
• Get rid of the target pseudo-selector, let the target element equal the effect target, either an Element or a CSSPseudoElement.

Start with the Web Animations Level 1 keyframe-effects section linked in the issue and review the existing discussion about nested pseudo-elements such as ::before::marker. Determine which target-element and target-pseudo-selector model the specification should define, then update the relevant wording once the working group reaches a decision.