No, because the provider would be in the position to change the SRI hashes. Sub Resource Integrity protects you against malicious CDNs and so on, but needs a non-compromised HTML page to provide correct hashes.
You could however probably provide a signed entry point via a webextension or so and a an audit trail via a trusted distribution plattform, like addons.mozilla.org. Are there apps which use a mechanism like this?
> No, because the provider would be in the position to change the SRI hashes. Sub Resource Integrity protects you against malicious CDNs and so on, but needs a non-compromised HTML page to provide correct hashes.
Can you not trust the originating site to serve non-compromised HTML if using HSTS and a trusted local certificate store (eliminating MITM as an attack vector)?
Not if the originating site is the potential attacker, which was the initial scenario: Protonmail sending you page that leaks your decrypted mail to them.
There was a way to make a permanent site installation on most browsers using HTML5 appcache, which even the web host couldn't update, but that API is deprecated in favor of the service worker's Cache API.
It's not clear to me that the Cache API offers the same level of security guarantee.
You could however probably provide a signed entry point via a webextension or so and a an audit trail via a trusted distribution plattform, like addons.mozilla.org. Are there apps which use a mechanism like this?