When I land on a page that's a PDF, I know certain things--I can easily save it and read it later. How do I know that? Not because I have read the PDF spec, or know that much about it, but because of my experience as a consumer of the web.
When I land on an arbitrary web-page, do I know the same thing? No. I don't know what the page is doing, I don't know what my browser will do when I try to save the page. When I save this page, I have the option to save HTML only, or a complete web page. Will the complete page actually work? I go into the source, and there's a link to the javascript (which is saved locally). Does rendering the page rely on that javascript? Does that javascript do xhr or fetch calls? Since it's Hacker News, I suspect the answer is no. However that's not inherent to the medium.
There are better ways to archive the content of even dynamic JS heavy pages, but they are not things that you learn as an average user of the web.