With people shifting their layouts over to Flexbox, relying on &quot;box-sizing: border-box&quot;, &quot;display: table-xxx&quot; and friends, older browsers will mess up the layout beyond belief. IIRC some older browsers even failed to apply an entire CSS file if they couldn&#x27;t parse a single rule - good luck with ::before, ::after, ::nth-child and friends.<p>CSS, unfortunately, isn&#x27;t really backwards compatible unless you invest a LOT of effort into testing and development.