Bramus Van Damme built a spec-compliant JavaScript implementation of the View Transition API that works across all browsers. It handles document.startViewTransition(), the ViewTransition class with its promises, and view transition types. Animations are skipped in unsupported browsers, but DOM updates work as expected.
bram.us/2026/03/11/view-transitions-mock-is-a-non-visual-polyfill-for-same-document-view-transitions