Bramus Van Damme explains the function the CSS Working Group adopted for capabilities you can’t test any other way. Checking @supports (display: flex) and (gap: 1em) only proves both parse, not that gap works in a flex container. The spec has two keywords so far: anchor-position-follows-transforms and single-axis-scroll-container.
bram.us/2026/08/27/feature-detecting-undetectable-css-features-with-supports-named-feature