View Transitions is an API that enables the creation of polished transitions. Web developers only need minimal effort to make transitions look nice. They can choose to use some default animation properties, or they can customize their own transition effects to achieve a desired transition experience. This is accomplished by leveraging user-agents’ ability to persist visual representations of rendered output (i.e. snapshots) and blend them with the live DOM state’s rendered output. The API also allows these animations to be customized via standard CSS animation properties.
Smooth page transitions can lower the cognitive load by helping users stay in context as they navigate from Page-A to Page-B, and reduce the perceived latency of loading. This feature provides a way to create an animated transition between two documents, without creating an overlap between the lifetime of each document. Although switching to an SPA allows developers to create transitions using existing technologies, such as CSS transitions, CSS animations, and the Web Animation API, it's something most developers and frameworks avoid, or only do in a limited fashion, because it's harder than it sounds.
Docs: https://github.com/WICG/view-transitions/blob/main/explainer.md
Samples: https://developer.chrome.com/docs/web-platform/view-transitions
Explainers: https://github.com/WICG/view-transitions/blob/main/explainer.md