I was just thinking about this. They're going for gold, but I think there's an easy solution that incrementally improves on the current situation without all the effort.

Right now you can download a webpage on most web browsers. It saves the html file and an associated folder. It would be cool if the web browsers placed that file and associated folder into a zip folder with a special file extension, and then the browser could read it on a double click as if you opened the html file.

Yes, I know it's limited. But dead simple. The format could evolve and get more bells and whistles over time.

Others mentioned epub. I do wish more sites offered epub downloads alongside or instead of pdf. Maybe browsers can do this, and maybe it'd be better than the simpler solution I suggested, but there are pros and cons to weigh here.