LibWeb: Update CSSRule parent style sheet whenever parent rule changes

Author: https://github.com/Calme1709
Commit: 89a08cb7cf
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5176
Reviewed-by: https://github.com/AtkinsSJ ✅

Libraries/LibWeb/CSS/CSSRule.cpp

@@ -53,6 +53,11 @@ void CSSRule::set_css_text(StringView)
 void CSSRule::set_parent_rule(CSSRule* parent_rule)
 {
     m_parent_rule = parent_rule;
+
+    if (parent_rule == nullptr)
+        set_parent_style_sheet(nullptr);
+    else
+        set_parent_style_sheet(parent_rule->parent_style_sheet());
     clear_caches();
 }