Related, but this is something that I would like if browsers gave more importance to.
Literally a few moments ago I was playing a video from a Nitter instance (Twitter proxy), and wanted to save it with Firefox but nope. Firefox already has the video there, I can see in the developer tools that it's playing from a blob. Even if the blob is then modified on-the-fly, why can't I right-click it and save that blob that you already have? If it's a stream, why can't I dump it? You are already playing it, you already have the bytes, you can "cache" the whole playback into a local file and then let me Right Click -> Save that "cache" file. Had to use `yt-dlp`.
Similar with bookmarks. When I bookmark something, it would be nice if the current state of the page were saved as a fallback (or at least have a checkbox for this behavior). You already have the rendered page right there. I can literally do `Ctrl+S` and save this page as it is right now, even with this half-typed comment. The code for saving the current state of a page is already there. And you can already link binary data to a bookmark (favicon), and text data (description, tags, etc). I wouldn't even mind if I have to give up the description field in exchange for a base64-encoded version of the bookmarked page, I'd be willing to accept that additional storage requirement.[1]
Sites like YouTube that actively work to prevent easy downloads and archiving might still need extensions like the one linked in the submission, but browsers don't make it easy to save some data that they already have, and it's difficult enough to contribute to (or patch for yourself) those massive codebases unless you are already familiar with them.
I'm willing to patch things for myself and repackage them for myself, but unfortunately for these things it's easier and more reliable long-term to just do workarounds.
(Disclaimer: I'm using Firefox.)
[1] EDIT: Having history enabled doesn't seem to do save an offline version, either.