LibWeb: Invalidate sibling styles on input element checked state change

Checkedness of an input element can influence sibling style, as well as
style of their children, when they use the `:checked` pseudo-class in
combination with a kind of sibling selector. That means its not
sufficient to just invalidate the input elements on style.

This is actually more commonly observable than one might expect, because
this pattern is often used as a JS-free toggle solution for things like
menus.

Userland/Libraries/LibWeb/HTML/HTMLInputElement.cpp

@@ -85,7 +85,12 @@ void HTMLInputElement::set_checked(bool checked, ChangeSource change_source)
         m_dirty_checkedness = true;
 
     m_checked = checked;
-    set_needs_style_update(true);
+
+    // This element's :checked pseudo-class could be used in a sibling's sibling-selector,
+    // so we need to invalidate the style of all siblings.
+    parent()->for_each_child([&](auto& child) {
+        child.invalidate_style();
+    });
 }
 
 void HTMLInputElement::set_checked_binding(bool checked)