The deleteRule() method of the CSSKeyframeRule interface deletes the CSSKeyFrameRule that matches the specified keyframe selector.

select

A string which contains the keyframe selector of the rule to be deleted, which must be:

• a comma-separated list of percentage values between 0% and 100%;
• or, the keywords from or to

Note that the number and order of the values in the specified keyframe selector must match those of the targeted keyframe rule(s). White-space is disregarded.

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 two rules.

After deleting a rule with deleteRule() the cssRules property returns a CSSRuleList containing one rule.

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

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

let myRules = document.styleSheets[0].cssRules;
let keyframes = myRules[0]; // a CSSKeyframesRule
keyframes.deleteRule("to");
console.log(keyframes.cssRules); // a CSSRuleList object with one rule