LibWeb: Mark elements for style update after their children change

Userland/Libraries/LibWeb/DOM/Element.cpp

@@ -389,4 +389,10 @@ int Element::client_height() const
     return box.absolute_rect().height();
 }
 
+void Element::children_changed()
+{
+    Node::children_changed();
+    set_needs_style_update(true);
+}
+
 }

Userland/Libraries/LibWeb/DOM/Element.h

@@ -122,6 +122,7 @@ public:
 
 protected:
     RefPtr<Layout::Node> create_layout_node() override;
+    virtual void children_changed() override;
 
 private:
     Attribute* find_attribute(const FlyString& name);