CSSStyleSheet.insertRule()

The CSSStyleSheet.insertRule() method inserts a new CSS rule into the current style sheet, with some restrictions.

Note: Although insertRule() is exclusively a method of CSSStyleSheet, it actually inserts the rule into CSSStyleSheet.cssRules — its internal CSSRuleList.

Syntax

stylesheet.insertRule(rule [, index])

Parameters

rule

A DOMString containing the rule to be inserted. What the inserted rule must contain depends on its type:

For rule-sets, both a selector and a style declaration.
For at-rules, both an at-identifier and the rule content.

index Optional

A positive integer less than or equal to stylesheet.cssRules.length, representing the newly inserted rule's position in CSSStyleSheet.cssRules. The default is 0. (In older implementations, this was required. See Browser compatibility for details.)

Return value

The newly inserted rule's index within the stylesheet's rule-list.

Exceptions

CSS has some intuitive and not-so-intuitive restrictions affecting where rules can be inserted. Violating them will raise an exception.

If index > CSSRuleList.length, the method aborts with an IndexSizeError DOMException.
If rule cannot be inserted at index 0 due to some CSS constraint, the method aborts with a HierarchyRequestError DOMException.
If more than one rule is given in the rule parameter, the method aborts with a SyntaxErrorDOMException.
If trying to insert an @import at-rule after a style rule, the method aborts with a HierarchyRequestError DOMException.
If rule is @namespace and the rule-list has more than just @import at-rules and/or @namespace at-rules, the method aborts with an InvalidStateError DOMException.

Examples

Inserting a new rule

This snippet pushes a new rule onto the top of my stylesheet.

myStyle.insertRule('#blanc { color: white }', 0);

Function to add a stylesheet rule

/**
 * Add a stylesheet rule to the document (it may be better practice
 * to dynamically change classes, so style information can be kept in
 * genuine stylesheets and avoid adding extra elements to the DOM).
 * Note that an array is needed for declarations and rules since ECMAScript does
 * not guarantee a predictable object iteration order, and since CSS is
 * order-dependent.
 * @param {Array} rules Accepts an array of JSON-encoded declarations
 * @example
addStylesheetRules([
  ['h2', // Also accepts a second argument as an array of arrays instead
    ['color', 'red'],
    ['background-color', 'green', true] // 'true' for !important rules
  ],
  ['.myClass',
    ['background-color', 'yellow']
  ]
]);
*/
function addStylesheetRules (rules) {
  var styleEl = document.createElement('style');

  // Append <style> element to <head>
  document.head.appendChild(styleEl);

  // Grab style element's sheet
  var styleSheet = styleEl.sheet;

  for (var i = 0; i < rules.length; i++) {
    var j = 1,
        rule = rules[i],
        selector = rule[0],
        propStr = '';
    // If the second argument of a rule is an array of arrays, correct our variables.
    if (Array.isArray(rule[1][0])) {
      rule = rule[1];
      j = 0;
    }

    for (var pl = rule.length; j < pl; j++) {
      var prop = rule[j];
      propStr += prop[0] + ': ' + prop[1] + (prop[2] ? ' !important' : '') + ';\n';
    }

    // Insert CSS Rule
    styleSheet.insertRule(selector + '{' + propStr + '}', styleSheet.cssRules.length);
  }
}

Polyfill

The below polyfill will correct the input of the arguments of insertRule() to standardize them in Internet Explorer 5–8. It supplements insertRule() with a function that separates the selector from the rules before sending the arguments to the default native insertRule().

(function(Sheet_proto){
  var originalInsertRule = Sheet_proto.insertRule;

  if (originalInsertRule.length === 2){ // 2 mandatory arguments: (selector, rules)
    Sheet_proto.insertRule = function(selectorAndRule){
      // First, separate the selector from the rule
      a: for (var i=0, Len=selectorAndRule.length, isEscaped=0, newCharCode=0; i !== Len; ++i) {
        newCharCode = selectorAndRule.charCodeAt(i);
        if (!isEscaped && (newCharCode === 123)) { // 123 = "{".charCodeAt(0)
          // Secondly, find the last closing bracket
          var openBracketPos = i, closeBracketPos = -1;

          for (; i !== Len; ++i) {
            newCharCode = selectorAndRule.charCodeAt(i);
            if (!isEscaped && (newCharCode === 125)) { // 125 = "}".charCodeAt(0)
              closeBracketPos = i;
            }
            isEscaped ^= newCharCode===92?1:isEscaped; // 92 = "\\".charCodeAt(0)
          }

          if (closeBracketPos === -1) break a; // No closing bracket was found!
            /*else*/ return originalInsertRule.call(
            this, // the sheet to be changed
            selectorAndRule.substring(0, openBracketPos), // The selector
            selectorAndRule.substring(closeBracketPos), // The rule
            arguments[3] // The insert index
          );
        }

        // Works by if the char code is a backslash, then isEscaped
        // gets flipped (XOR-ed by 1), and if it is not a backslash
        // then isEscaped gets XORed by itself, zeroing it
        isEscaped ^= newCharCode===92?1:isEscaped; // 92 = "\\".charCodeAt(0)
      }
      // Else, there is no unescaped bracket
      return originalInsertRule.call(this, selectorAndRule, "", arguments[2]);
    };
  }
})(CSSStyleSheet.prototype);

Specifications

Specification
CSS Object Model (CSSOM) (CSSOM) # dom-cssstylesheet-insertrule

Browser compatibility

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	WebView Android	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet
`insertRule`	1	12	1	9	≤12.1	1	1	18	4	≤12.1	1	1.0
`index_parameter_optional`	1	12	55	No	15	1	1	18	55	14	1	1.0

Legacy browser support

To support Internet Explorer 8 and below, use: addRule(selector, rule [, index]);. Example: addRule('pre', 'font: 14px verdana'); // add rule at end

Also note the non-standard removeRule() and .rules instead of deleteRule() and .cssRules, respectively.