Let CSSPageRule inherit from CSSGroupingRule instead of CSSRule. The spec [1] says that CSSPageRule interface should inherit from CSSGroupingRule, but Blink inherits from CSSRule instead. In order to support @page margins [2] (CSSMarginRule), CSSPageRule needs to support child rules. [1] https://drafts.csswg.org/cssom/#the-csspagerule-interface [2] https://drafts.csswg.org/css-page-3/#syntax-page-selector
In order to access CSSMarginRule via CSSOM, CSSPageRule needs to support child rules.