The test case creates an animation with all properties by implicit keyframes. I tried to make it simpler: only create `opacity: ['initial']`, and I still can reproduce this issue. However, [`opacity` shouldn't be applied to `::marker` pseudo element](https://drafts.csswg.org/css-pseudo-4/#marker-pseudo), so I suspect that's why we can not get an underlying value for opacity on '::marker'.
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The test case creates an animation with all properties by implicit keyframes. I tried to make it simpler: only create `opacity: ['initial']`, and I still can reproduce this issue. It seems [`opacity` shouldn't be applied to `::marker` pseudo element](https://drafts.csswg.org/css-pseudo-4/#marker-pseudo), so I suspect that's why we can not get an underlying value for `opacity` on `::marker`.