Adding support of Internet Message Body Format (a.k.a. MIME) to browsers [1].
MIME is a format that can contain html/css/script/images/etc in single file (or stream).
Thus the whole web application can be served as a single stream by the server.
Yet emails (that are MIME files) can be opened by browsers as natively supported documents.
[1] MIME : https://tools.ietf.org/html/rfc2045
You might be interested in Web Packaging: https://github.com/WICG/webpackage