The read-only CSSStyleSheet property ownerRule returns the CSSImportRule corresponding to the @import at-rule which imported the stylesheet into the document. If the stylesheet wasn't imported into the document using @import, the returned value is null.

A CSSImportRule corresponding to the @import rule which imported the stylesheet into the document. If the stylesheet wasn't imported into the document using @import, the returned value is null.

This snippet of code looks for rules which were not imported into the document using an @import at-rule.

const ruleList = document.styleSheets[0].cssRules;

for (const rule of ruleList) {
  if (!rule.ownerRule) {
    /* rule is not imported */
  }
}

This snippet obtains a reference to the stylesheet associated with the @import and processes it in some manner:

const ruleList = document.styleSheets[0].cssRules;

for (const rule of ruleList) {
  if (rule.ownerRule) {
    checkStylesheet(rule.ownerRule.styleSheet);
  }
}

• CSS Object Model
• Using dynamic styling information