Currently, @supports and CSS.supports only support property: value pairs. The only way for web developers to detect support for a certain descriptor in an at-rule is to use JavaScript. For example:

let e = document.createElement("style");
e.textContent = "@font-face { font-display: swap; }";
document.documentElement.appendChild(e);
let isFontDisplaySupported = e.sheet.cssRules[0].cssText.indexOf("font-display") != -1;

(from https://bugzilla.mozilla.org/show_bug.cgi?id=1296373#c6)

It would be good if CSS can test whether a user agent supports a descriptor: value pair.

See also a related issue in Chromium: https://bugs.chromium.org/p/chromium/issues/detail?id=600137

Tagged as "unknown/future spec" for now, because I'm not sure if we should extend @supports (which is in css-conditional), or develop a new mechanism for it.