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.”
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.
Feelings and promises are each one thing. Reality is another. PDF doesn't even look "the same" today. I have serious questions about how often folks who think that PDF is reliably consistent from system to system step outside their bubble and just how diverse their setups are that they're testing on.
> is HTML the same way?
Well the status-quo for copy-and-paste in HTML isn't dogshit, it's comparatively trivial to find and use tools that can thoroughly and exhaustively search your collection (or even write your own), and HTML is a dead simple plain-text format that if worst comes to worst you can read with your eyes (unlike needing to run a bunch of inscrutable code from a PostScript subset through an interpreter before you can do anything with it). So, no, I wouldn't call them the same.
I’d much rather have something akin to the CHM files where everything I need is in one file, easy to analyze, and has good readers.
Re. self-contained HTML (and slightly off-topic), look at TiddlyWiki, which contains data/code/layout all in one interactive, local or hosted HTML. Extensibility, plugins, and community of contributors are some key highlights, among others.