Really impressive demo. First, this is for single-page applications. Second, there is an API for multi-page applications. Check it out in Chrome Canary and look at the code. I discussed this with my team yesterday. The demo is built on Astro. All that is shipped to the browser is 301kB. Of that 291kB is images. Less than 5.5kB for the document, CSS, and JS. CSS is powering the transitions and only a bit of JS intercepts the navigation event, loads the fragment of HTML, injects it into the DOM, and adds the necessary classes to trigger the animations.