The CSSStyleDeclaration.parentRule read-only property returns a CSSRule that is the parent of this style block, e.g. a CSSStyleRule representing the style for a CSS selector.

The CSS rule that contains this declaration block or null if this CSSStyleDeclaration is not attached to a CSSRule.

The following JavaScript code gets the parent CSS style rule from a CSSStyleDeclaration:

const declaration = document.styleSheets[0].rules[0].style;
const rule = declaration.parentRule;