There was a time when Opera was not a google chrome skin and offered indexed full page search of local cache of visited page. This was incredibly useful and practical. It also had the option to save the whole web page in a single file.

Sadly this kind of feature disappeared when opera moved to a quarterly profit based strategy.

The kind of feature you want is achievable today but requires a set of tools beyond a web browser and some human work. It's also a bit tedious to maintain up to date.