The obsolete CSSStyleSheet interface's addRule() legacy method adds a new rule to the stylesheet. You should avoid using this method, and should instead use the more standard insertRule() method.

selector

A string specifying the selector portion of the CSS rule. The default is the string undefined.

styleBlock

A string indicating the style block to apply to elements matching the selector. The default is the string undefined.

index Optional

An optional index into the stylesheet's CSSRuleList at which to insert the new rule. If index is not specified, the next index after the last item currently in the list is used (that is, the value of cssStyleSheet.cssRules.length).

Always returns -1.

Note that due to somewhat esoteric rules about where you can legally insert rules, it's possible that an exception may be thrown. See insertRule() for more information.

This method is implemented by browsers by constructing a string using the template literal `${selector}{${styleBlock}}`, then passing it into the standard insertRule() method.

Therefore, given existing code such as the following:

cssStyleSheet.addRule(selector, styles, 0);

You can rewrite this to use the more standard insertRule() like this:

cssStyleSheet.insertRule(`${selector} {${styles}}`, 0);

• CSS Object Model
• Using dynamic styling information