It's easier to see if you play with it on the actual site rather than look at the images and try to deduce what could be different. It's not really related to things like order or positioning of boxes, that's an extremely easy problem to solve. Some of it is loading progressively space saving assets e.g. the logo goes from "${icon} airbnb" to "${icon}" to "" depending on how much space there is, the search bar simplifies so you have space to actually type, and some content border elements are removed so there is more room for the actual content e.g. rather than blindly always show content boxes with rounded borders, spacing, and other attributes you can gain a lot of space back on small displays by stretching the container's content to fill that part of the screen completely. This is particularly useful if you combine this with getting rid of certain UI elements from earlier.

HN also does what's described at the end using media queries - if you make the page small your topbar fills the entire top and changes element layout while the post content area fills the entire rest of the screen.

And if you make your page slightly bigger than the threshold for triggering HN's "mobile view", there's still padding in the top bar but the page is 53 pixels too wide, and you have to scroll horizontally. Hacker News is an example of why we don't use media queries to hack in a responsive layout. Websites are responsive by default, unless you're using <table> layouts (as HN does) or the <div>pocalypse (as every other website seems to, these days). Building different versions of your site for different widths is not a solution.

The more subtle things you describe seem quite sensible. I'll probably steal those ideas, if it comes up.

> Building different versions of your site for different widths is not a solution.

I think about it as building different sites for different form factor. What you do on a phone is different from what you do on a desktop. Or a tablet. I don't like using the amazon website on mobile because it's so cluttered, when what I want is usually searching for a product, or checking my cart for a product. I'm not managing my account in there, nor do I want alternative recommendations.

What you do on a phone is different from what you do on a desktop. What if I want to do the "mobile" activities on a desktop, or zoomed in enough to trigger the viewport changes? What if I want to do the "desktop" activities on a mobile? If you want to make a separate mobile site, make a separate mobile site: don't use effective screen resolution as a proxy.

Yeah, you can definitely fuck it up, but the same can be said for flex/grid/masonry layouts yet you wouldn't say "and grid spanning errors on resize are why we never use grid". HN does a lot "wrong" in its design (like the aforementioned table hell) but that doesn't mean every mistake it makes is a universal thing to avoid and inherent to the way it was implemented. Reddit, YouTube, Facebook, Instagram, Netflix, X, Amazon, and most modern sites do the same kind of content box snapping on either the header or content area (or both) via breakpoints. Most do it a lot better than HN :). The top site I know that doesn't really do that (but it does do the minor breakpoint things) is Yahoo.com.