The appendRule() method of the CSSKeyframeRule interface appends a CSSKeyFrameRule to the end of the rules.

rule

A string containing a keyframe rule.

None (undefined).

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. Returning the cssRules property would return a CSSRuleList containing one rule.

After appending another rule with appendRule the cssRules property returns a CSSRuleList containing two rules.

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

let myRules = document.styleSheets[0].cssRules;
let keyframes = myRules[0]; // a CSSKeyframesRule
keyframes.appendRule("to {transform: translateX(100%);}");
console.log(keyframes.cssRules); // a CSSRuleList object with two rules