Conversation

Replace HTML/CSS/JavaScript with mini-languages for parsing, FRP/dataflow, and constraint-based layout/rendering;
Replace cookies and login forms with local storage and HTTP auth, but always require user permissions;
Replace the same-origin policy so you can fetch anything from anywhere but it can't use local storage or auth, or you can punch a hole in your layout for remote content to run isolated from yours;
...too much to fit in a toot