My site looks fine in Chrome but breaks in Safari / on iPhone. Why?

Safari and iOS handle a few things differently: 100vh changes as the toolbar shows/hides, flexbox gap needed a fallback on older versions, backdrop-filter needs the -webkit- prefix, sticky positioning and :has() behave differently, and form inputs are styled hard. We reproduce on real iOS Safari (not just a resized desktop window) and add the right fallbacks and prefixes so the page matches Chrome.