The potential to freeze an HTML page in time with minimal changes at render time is already there. [0] Such an ability can even be baked directly into the rendered HTML page so the viewer would be able to download a copy of the page as it is seen at a given time. Other archiving facilities, such as archive.org, take static snapshots of accessible pages if allowed by the publisher of the page and requested by anyone who wants to make that snapshot.
My point is that it is possible to achieve in principle and in practice, albeit that might be practiced as often as one would like to see.
I like SingleFile, but it's not perfect. It usually works just fine, but will occasionally drop the ball depending on the type of JavaScript on the page.
For example, I once backed up a page using it, and while it got all the content, it did not grab the JavaScript necessary for the images to display correctly.
My point is that it is possible to achieve in principle and in practice, albeit that might be practiced as often as one would like to see.
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[0] See SingleFile by gildas at https://addons.mozilla.org/en-US/firefox/addon/single-file/: “Save an entire web page—including images and styling—as a single HTML file.”