A proposed API for shared storage in the browser along with a working polyfill (and browser settings emulator).

(async () => {
let status, attempt, data;

({status, attempt, data} = await SharedStorage.set({
  namespace: 'test', data: 'abc'
}));
assert(
  status === 'success' && attempt === 'set' && data === undefined,
  'Passed setting'
);

({status, attempt, data} = await SharedStorage.get({namespace: 'test'}));
assert(
  status === 'success' && attempt === 'get' && data === 'abc',
  'Passed getting'
);

// Stop-gap utility until polyfill can farm out data to other of its
//  domains and/or browsers can determine any quotas
const {maxRemaining} = await SharedStorage.getMaxRemaining();
assert(typeof maxRemaining === 'number');
})();

• Operating on same files with different viewers/editors (e.g., word processing documents, SVG editors, etc.)
• Decentralized, offlineable social media
• Email, chat, and feed subscription systems
• Hierarchical pseudo-file system available to websites
• Plugin architecture for any web app (sites can use shared storage to save their plugin URLs while the main app uses postMessage to communicate with those apps)
• Web Intents replacement (see "Plugin architecture" just above)

1. Preferences page currently only shows preferences as set by other sites; make editable
2. Get origin-specific namespacing types working:
3. origin -> namespace (written by single origin but read-only to others) - E.g., for .gov practicing Open Data
4. namespace -> origins (written by each origin and read-only to others) - E.g., sites enumerating their details for a shared namespace (could be chaotic if versioned/interpreted differently, however). Useful, e.g., for delivery of emails
5. origin -> namespace -> origins (main area written by single origin, but other origins being allowed to add to namespaces; read-only to others) - E.g., for plugins to a particular site (e.g., handling sites of .gov data)
6. Consider key, removeItem, clear, length; renaming get/set to getItem/setItem
7. Implement shared IndexedDB version