To recap, View Transitions animate snapshots and because of that things like a changing border won’t nicely animate but they will fade from the old to the new snapshot. So instead of creating an animation on the ::view-transition-group pseudo, Martin’s approach is to show only the new state of the box – through the ::view-transition-new pseudo – and rely on CSS transitions to have it transition the border with a duration that is set to the same duration + timing-function as the View Transition.