Ahmad Shadeed documents a bonafide CSS trick from the Facebook CSS codebase. The idea is that when an element is the full width of the viewport, it doesn’t have any border-radius. But otherwise, it has 8px of border-radius. Here’s the code:
border-radius: max(0px, min(8px, calc((100vw – 4px – 100%) * 9999)));
One line! Super neat. The guts of it is the comparison between 100vw and 100%. Essentially, the border-radius comes out to 8px most of the time. But if the component becomes the same width as the viewport (within 4px, but I’d say that part is optional), then the value of the border-radius becomes 0px, because the equation yields a negative (invalid) number.
The 9999 multiplication means that you’ll never get low-positive numbers. It’s a toggle. You’ll either get 8px or 0px and nothing in between. Try removing that part, resizing the screen, and seeing it sorta morph as the viewport becomes close to the component size:
Why do it like this rather than at a @media query? Frank, the developer behind the Facebook choice, says:
It’s not a media query, which compares the viewport to a fixed width. It’s effectively a kind of container query that compares the viewport to its own variable width.
This is a more general purpose solution as it doesn’t need to know the size of the card. A media query would be dependent on the width of the card.