The read-only cssRules property of the CSSKeyframeRule interface returns a CSSRuleList containing the rules in the keyframes at-rule.

A CSSRuleList.

The CSS includes a keyframes at-rule. This will be the first CSSRule returned by document.styleSheets[0].cssRules. myRules[0] returns a CSSKeyframesRule object. The cssRules property returns a CSSRuleList containing two rules.

@keyframes slidein {
  from {
    transform: translateX(0%);
  }

  to {
    transform: translateX(100%);
  }
}

let myRules = document.styleSheets[0].cssRules;
let keyframes = myRules[0]; // a CSSKeyframesRule
console.log(keyframes.cssRules); // a CSSRuleList object with two rules