Allow view-transitions to generate a nested pseudo-element tree rather than a flat one. This allows the view transition to appear more in line with its original elements and visual intent, as it enables clipping, nested 3D transforms, and proper application of effects like opacity, masking and filters.
Flat-tree view transitions, as available today, are limited: they don't work correctly and don't provide the full set of expression features with regards to clipping, 3D transforms, masking and filter effects, as those work differently on a flat tree and on a hierarchical tree. By representing the captured elements by a nested tree we let authors apply those CSS features correctly on the view transition pseudo-elements, resulting in view transitions that are both more visually correct and contain the entire set of CSS visual expressions.
Explainers: https://github.com/WICG/view-transitions/blob/main/nested-explainer.md