CSSRule.parentRule
  The parentRule property of the CSSRule
  interface returns the containing rule of the current rule if this exists, or otherwise
  returns null.
Value
A CSSRule which is the type of the containing rules. If the current rule is inside a media query, this would return CSSMediaRule. Otherwise it returns null.
Examples
@media (min-width: 500px) {
  .box {
    width: 100px;
    height: 200px;
    background-color: red;
  }
  body {
    color: blue;
  }
}
let myRules = document.styleSheets[0].cssRules;
let childRules = myRules[0].cssRules;
console.log(childRules[0].parentRule); // a CSSMediaRule
Specifications
| Specification | 
|---|
| CSS Object Model (CSSOM) # dom-cssrule-parentrule | 
Browser compatibility
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