The insertRule method inserts a new style rule into the current style sheet.
rule is a string containing the rule to be inserted (selector and declaration)
index is a number representing the position to be inserted
For rule sets this contains both the selector and the style declaration. For at-rules, this specifies both the at-identifier and the rule content.
DOM cssRule Object
The cssRule object represents a single CSS style rule. These rules may be a part of a stylesheet or they may be placed in-line with the individual nodes in the HTML or XML document. Each stylesheet object exposes an array of the cssRules that make it up, and you can also get to the rules on individual elements by using the element.style property.
cssText returns the actual text of the style rule. parentStyleSheet returns the stylesheet object in which the current rule is defined. selectorText gets/sets the textual representation of the selector for the rule set. style returns the declaration block for the current style.