Ask HN: Browser GUI

How would you go about writing a GUI that would run in most browsers on all platforms that support those browsers, even if offline? An example might be a Spreadsheet application that would still work offline but have extra abilities online (cloud storage, collaboration etc). The ability to display charts and simple animations/sounds would be a requirement.

Some other things to discuss: 1. How would this picture change if offline ability was not a requirement? 2. What if the application was very graphics/sound intensive (game)?

(Hope this isn't too general a question)