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@thomasp You do realize that this is how respectable password managers do it; They decrypt locally in javascript, never sending the password away.
As for the "guarantee" of the page source code, it would be on you. I understand that you may be reluctant to take that responsibility, but you are also responsible for the source code of Vivaldi, which has a similar threat model.
Not only that, but the access could be controlled by a setting: If I don't want access to passwords on the web, I remove the setting for password access on the web. Of course at this point you can start chasing your tail by arguing that hackers could change the settings too, etc. But if those settings can only be done in the Vivaldi browser, you're back to the same threat model you have now, and everybody is happy (ier).
In any case, it might be better to let users decide what is "sensitive" information. If you don't want to take responsibility for the password, that is understandable, but other aspects could be made available.
PS: Since you mention it, I just wanted to say that the dual password setup is cheap. There are better ways to manage dual access with a single password. Most password managers do.
