CSSRuleList

A CSSRuleList represents an ordered collection of read-only CSSRule objects.

While the CSSRuleList object is read-only, and cannot be directly modified, it is considered a live object, as the content can change over time.

To edit the underlying rules returned by CSSRule objects, use CSSStyleSheet.insertRule() and CSSStyleSheet.deleteRule(), which are methods of CSSStyleSheet.

This interface was an attempt to create an unmodifiable list and only continues to be supported to not break code that's already using it. Modern APIs represent list structures using types based on JavaScript arrays, thus making many array methods available, and at the same time imposing additional semantics on their usage (such as making their items read-only).

These historical reasons do not mean that you as a developer should avoid CSSRuleList. You don't create CSSRuleList objects yourself, but you get them from APIs such as CSSStyleSheet.cssRules and CSSKeyframesRule.cssRules, and these APIs are not deprecated. However, be careful of the semantic differences from a real array.

body {
  font-family:
    system-ui,
    -apple-system,
    sans-serif;
  margin: 2em;
}

.container {
  display: grid;
  grid-template-columns: repeat(auto-fill, 200px);
}

.container > * {
  background-color: #3740ff;
  color: #fff;
}

let myRules = document.styleSheets[0].cssRules;
console.log(myRules);
console.log(myRules.length);
console.log(myRules[0]);