Experimental browser API — essentially Hotwire streams baked into the web platform. Uses HTML template tags with start/end markers to declaratively replace, append to, or remove named regions of the DOM as HTML streams down. Supports re-binding identifiers for continuous appends. Works great for out-of-order streaming of slow dashboard widgets. Currently JavaScript-only (few lines of code to wire it up, easily combined with View Transitions). Ryan's speculation: browsers could adopt this so a link or form submit with an opt-in attribute posts to the server, the server returns only the changed fragment, and the browser applies it — server-driven soft navigation with zero JavaScript. His demo email-signup form implements a polyfill and the whole UI runs on 614 bytes of JS. Chrome preview available behind flags; Ryan plans to push for browser adoption as his first browser API.