I’ve been thinking for awhile about save mechanisms and back buttons. I’ve never tried to modify an existing game, so have no idea about that. If you wanted to code those functions, though, you could.
The most basic temporary save states that people use for their WIPs (which rely on setting CS variables as of some point in the game) are not permanent if someone closes their browser. But between those, achievement workarounds (game completed achievements could unlock New Game+ content) and the chapter save points implemented by Hosted games like @JimD’s Zombie Exodus, you can give the player some control over where to start over.
If you want to let the player go back a page to re-read a passage, or even to reconsider a choice they didn’t understand (maybe you could limit this option? Make it a predictive superpower or an intuition skill?) you could add a label to the top of each page. Each page_break would then be a choice, “Continue reading” (goto_scene “next scene name”) or “Go back a page” (goto_scene “previous scene name”.) Being label-based it would take more time to code, but would work.
Personally, I’d use this to allow re-reading (if they need information from the last page, I gave it to them for a reason) but wouldn’t let people re-choose options unless their chances to do so were very restricted. Players shouldn’t mistake the options more than once or twice a game unless you failed to explain them and deserve to get bad reviews, or they’re not paying attention to the clues you handed them.