View Transitions
A browser API that animates the change between two states of a page, or between two pages, without the site having to hold both in memory itself.
The browser takes a picture of the old state, lets the new one be built, and animates between the two. Elements that should look continuous across the change are matched by name, which is how a thumbnail can appear to grow into the article image it links to.
The version that works across navigations is the interesting one, because it removes the reason many sites became single-page applications in the first place: keeping the transition smooth. A plain multi-page site can now have the animation without the router, the client-side state, and the hydration bill.
It degrades to nothing, which is the point. A browser without support performs the same navigation without the animation, so it can be added as a refinement rather than budgeted for as a dependency — check its Baseline status for the flavour you need, since same-document and cross-document did not arrive together.