They had some kind of intermediate representation of page renders that was efficiently cached on disk so that it made zero network requests on history navigation. I suspect this same approach also played a part in facilitating the fulltext history search feature I've also never seen in a browser since.

I'm guessing with the way web standards have evolved & become more complex this might actually be impossible to do today while remaining compliant - honestly give me non-compliance though.

True! Came to post the same thing - one of my favourite feature of Opera Presto engine was how all the websites in your history was also "indexed" locally, so that you could do a simple keyword search on "History" to find the web page you wanted to re-visit. It was fast and accurate and made it a breeze to find any site in that you had browsed and was still cached, and it was an incredibly useful feature.

Yeah, I don't know, I don't see how you can't pause execution and store the entire interpreter state and DOM somewhere. Maybe it's just that nobody cares enough to go through all the effort?