There are a lot of other implementations of this idea that don't necessarily rely on trust-on-first-use. The securedrop team explicitly includes malicious JS served by the primary-domain in the threat-model and made WEBCAT[0] as an outcome of that research. Their article on webcrypto is much better than this one.
replyThe solution obviously is to go out-of-band:
> When a user visits a website that has enrolled in WEBCAT, before the site can load the content is checked against a signed manifest to ensure that it has not been tampered with (more on enrollment later). If everything checks out, the page loads normally. If, however, any content does not match what’s expected, the page load is aborted and a warning is displayed, protecting the user from potentially malicious content before it can execute.
[0]: https://securedrop.org/news/introducing-webcat-web-based-cod...
[1]: https://securedrop.org/news/browser-based-cryptography/