In the case I was referring, we loaded files as blobs in the browser's local storage, allowing us to run a web worker as a persistent local cache server.

This involved managing Firefox's admirable response doctoring policies, though.

All other browsers would let a web worker modify the incoming response to a request before forwarding it to an iframe, but FF absolutely refused it and required that the modified response get constructed anew in its local context, meaning locally created or injected content could not be mistakenly trusted as origin content.