The read-only supportsText property of the CSSImportRule interface returns the supports condition specified by the @import at-rule.

A string, or null.

The document's single stylesheet contains three @import rules. The first declaration imports a stylesheet if display: flex is supported. The second declaration imports a stylesheet if the :has selector is supported. The third declaration imports a stylesheet without a supports condition.

The supportsText property returns the import conditions associated with the at-rule.

@import url("style1.css") supports(display: flex);
@import url("style2.css") supports(selector(p:has(a)));
@import url("style3.css");

const myRules = document.styleSheets[0].cssRules;
console.log(myRules[0].supportsText); // returns `"display: flex"`
console.log(myRules[1].supportsText); // returns `"selector(p:has(a))"`
console.log(myRules[2].supportsText); // returns `null`

• Using feature queries
• @import and @supports