Returns the containing rule, otherwise null.
Returns the containing rule, otherwise null. E.g. if this rule is a style rule
inside an @media
block, the parent rule would be that CSSMediaRule.
MDN
parentStyleSheet returns the stylesheet object in which the current rule is defined.
parentStyleSheet returns the stylesheet object in which the current rule is defined.
MDN
https://drafts.csswg.org/cssom/#the-cssrule-interface