You send a friend a link, they open it, and suddenly buttons are floating in the wrong place, the layout is off, and the colors look wrong. You're both looking at the exact same URL, yet the pages look nothing alike. This happens more often than you'd think.
Every webpage is built on the same set of universal coding standards, but browsers like Chrome, Safari, Edge, and the default browsers baked into phones don't all implement those standards identically — especially for newer or less common features. The result is that the same code can render with subtle (or not-so-subtle) differences depending on which browser opens it.
Even within the same browser, an old version may simply not recognize some of the newer styling techniques a site uses. When a browser hits code it doesn't understand, it typically just ignores it — which is exactly how pages end up looking incomplete or visually broken.
Web developers typically use a browser compatibility checker to look up how well a given CSS property is supported across major browsers before shipping. That way they catch problems like "this effect won't render on an older version of Safari" in advance, instead of waiting for users to report a broken page.
So next time the same page looks different on two different screens, it's probably not the site that's broken — try opening it in an updated browser first. More often than not, that fixes it right away.