I can’t speak for Gecko or Blink/Chromium, but I don’t think this would be difficult to implement in a WebKit browser as long as some limits are involved.
Just save the tab’s contents in a webarchive file, then when restoring load that webarchive and inject some JavaScript that makes any kind of interaction aside from scrolling load the page’s live counterpart (with a confirmation prompt, perhaps).
This would also come with the advantage of the cached version showing up in browser history for “free”, so if you inadvertently go to the live page undoing it is as easy as hitting the back button, and if you decide you want to preserve the cached state indefinitely all the browser has to do is copy the webarchive file out of temporary storage to wherever the user indicates.
Just save the tab’s contents in a webarchive file, then when restoring load that webarchive and inject some JavaScript that makes any kind of interaction aside from scrolling load the page’s live counterpart (with a confirmation prompt, perhaps).
This would also come with the advantage of the cached version showing up in browser history for “free”, so if you inadvertently go to the live page undoing it is as easy as hitting the back button, and if you decide you want to preserve the cached state indefinitely all the browser has to do is copy the webarchive file out of temporary storage to wherever the user indicates.