Browser compatibility is the degree to which a website behaves the same across different browsers and devices. A page that renders perfectly in Chrome might shift its layout in Safari or lose an animation in Firefox. Good compatibility means every visitor gets a working, consistent experience regardless of what they open the site in.
The reason this is even a problem comes down to engines. Google Chrome runs on Blink, Safari runs on WebKit, and Firefox runs on Gecko. Think of three chefs cooking from the same recipe but with slightly different ovens. The dish is recognisable in each kitchen, yet the timing and finish differ. So a CSS feature might be fully supported in one browser, partly supported in another, and missing in a third, especially on older versions.
The practical fix is testing and graceful fallbacks. You check the browsers your visitors actually use, then patch the gaps. For an unsupported feature, a small polyfill can fill in the missing capability so an older browser still behaves. The goal is not pixel-identical everywhere, it is reliably usable everywhere. Your analytics tell you which browsers matter: if barely anyone visits in an old version of Edge, you do not waste a day chasing a bug there. And the cost of skipping this work is concrete. A date picker that silently fails in Safari, a payment button that does nothing in an older browser, a layout that overlaps on one phone, each one is a customer who leaves and a sale you never see.
At TopDevs we test across the major browsers on real devices before launch, so a client never finds out about a Safari bug from an annoyed customer.
