CSSStyleSheet.insertRule()

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

More specifically, though insertRule() is exclusively a method of the CSSStyleSheet, it actually inserts the rule into CSSStyleSheet.cssRules, the CSSRuleList therein.

What the rule must contain depends on its type: For rule-sets the rule specifies both the selector and the style declaration. For at-rules, the rule specifies both the at-identifier and the rule content.

Syntax

stylesheet.insertRule(rule, index)

Parameters

• rule is a DOMString containing the rule to be inserted,
  ...where rule specifies selector and declaration, or at-identifier and rule content.
• index is an unsigned int representing the position in CSSStyleSheet.cssRules to insert at,
  ...where index-0 is first rule, and index-max is just after last rule and is the same as length of CSSStyleSheet.cssRules.

Note: index is now formally required, and omitting it may cause a warning or error. For example:

• Chrome: " [Warning-icon]  Calling CSSStyleSheet.insertRule() with one argument is deprecated. Please pass the index argument as well"

Return value

The index within the style sheet's rule-list of the newly inserted rule.

Restrictions

CSS stylesheet rule-lists have a number of intuitive and not-so-intuitive restrictions affecting how and where rules can be inserted. Violating these will likely cause an error raised as a DOMException 

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

Examples

Example 1

// push a new rule onto the top of my stylesheet
myStyle.insertRule("#blanc { color: white }", 0); 

Example 2

/**
 * Add a stylesheet rule to the document (may be better practice, however,
 * 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 afford a predictable object iteration order and since CSS is 
 * order-dependent (i.e., it is cascading); those without need of
 * cascading rules could build a more accessor-friendly object-based API.
 * @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'),
      styleSheet;

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

  // Grab style sheet
  styleSheet = styleEl.sheet;

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

Specifications

Browser compatibility

Legacy browser support

• Internet Explorer - pre v9

  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.

See also

• CSSStyleSheet.deleteRule
• Cross-Browser CSS-rules ordering (CSS1)
• Quirksmode - CSS