supports-feature-query-progressive-enhancement

CSS feature queries enable progressive enhancement by wrapping modern properties in @supports blocks, ensuring legacy browsers receive structural fallbacks while modern engines render advanced styling.

The best way to provide fallbacks for modern CSS features is using structural selectors like :has() and :nth-of-type() within @supports not blocks to approximate modern behavior for legacy browser compatibility.

@supports not works for legacy browser compatibility by explicitly checking for the absence of a CSS feature, allowing you to isolate and apply structural fallback styling only when modern rendering engines lack that capability.

You can use calc-size() and anchor positioning with older browsers by gating these advanced layout features within feature queries, which ensures browsers lacking support receive a graceful, usable fallback interface.

You need structural fallbacks for CSS layout features when design intent must be preserved across diverse rendering engines, ensuring the interface remains functional and visually coherent even without modern property support.