View Transition Name Required

Elements participating in view transitions need view-transition-name.

Incorrect (no transition name):

document.startViewTransition(() => {
  targetImg.src = newSrc;
});

Correct (transition name assigned):

sourceImg.style.viewTransitionName = "card";
document.startViewTransition(() => {
  sourceImg.style.viewTransitionName = "";
  targetImg.style.viewTransitionName = "card";
});