LibWeb: Associate form elements with a form in parsing and dynamically

This makes it available for all form associated elements and not just
select and input elements. It also makes it more spec compliant,
especially around the form attribute.

The main thing missing is re-associating form elements with a form
attribute when the form attribute changes or an element with an ID
is inserted/removed or has its ID changed.

Userland/Libraries/LibWeb/HTML/FormAssociatedElement.cpp

@@ -45,4 +45,74 @@ bool FormAssociatedElement::enabled() const
     return true;
 }
 
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#association-of-controls-and-forms:nodes-are-inserted
+void FormAssociatedElement::inserted()
+{
+    HTMLElement::inserted();
+
+    // 1. If the form-associated element's parser inserted flag is set, then return.
+    if (m_parser_inserted)
+        return;
+
+    // 2. Reset the form owner of the form-associated element.
+    reset_form_owner();
+}
+
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#association-of-controls-and-forms:nodes-are-removed
+void FormAssociatedElement::removed_from(DOM::Node* node)
+{
+    HTMLElement::removed_from(node);
+
+    // 1. If the form-associated element has a form owner and the form-associated element and its form owner are no longer in the same tree, then reset the form owner of the form-associated element.
+    if (m_form && &root() != &m_form->root())
+        reset_form_owner();
+}
+
+// https://html.spec.whatwg.org/multipage/form-control-infrastructure.html#reset-the-form-owner
+void FormAssociatedElement::reset_form_owner()
+{
+    // Although these aren't in the "reset form owner" algorithm, these here as they are triggers for this algorithm:
+    // FIXME: When a listed form-associated element's form attribute is set, changed, or removed, then the user agent must reset the form owner of that element.
+    // FIXME: When a listed form-associated element has a form attribute and the ID of any of the elements in the tree changes, then the user agent must reset the form owner of that form-associated element.
+    // FIXME: When a listed form-associated element has a form attribute and an element with an ID is inserted into or removed from the Document, then the user agent must reset the form owner of that form-associated element.
+
+    // 1. Unset element's parser inserted flag.
+    m_parser_inserted = false;
+
+    // 2. If all of the following conditions are true
+    //    - element's form owner is not null
+    //    - element is not listed or its form content attribute is not present
+    //    - element's form owner is its nearest form element ancestor after the change to the ancestor chain
+    //    then do nothing, and return.
+    if (m_form
+        && (!is_listed() || !has_attribute(HTML::AttributeNames::form))
+        && first_ancestor_of_type<HTMLFormElement>() == m_form.ptr()) {
+        return;
+    }
+
+    // 3. Set element's form owner to null.
+    set_form(nullptr);
+
+    // 4. If element is listed, has a form content attribute, and is connected, then:
+    if (is_listed() && has_attribute(HTML::AttributeNames::form) && is_connected()) {
+        // 1. If the first element in element's tree, in tree order, to have an ID that is identical to element's form content attribute's value, is a form element, then associate the element with that form element.
+        auto form_value = attribute(HTML::AttributeNames::form);
+        root().for_each_in_inclusive_subtree_of_type<HTMLFormElement>([this, &form_value](HTMLFormElement& form_element) mutable {
+            if (form_element.attribute(HTML::AttributeNames::id) == form_value) {
+                set_form(&form_element);
+                return IterationDecision::Break;
+            }
+
+            return IterationDecision::Continue;
+        });
+    }
+
+    // 5. Otherwise, if element has an ancestor form element, then associate element with the nearest such ancestor form element.
+    else {
+        auto* form_ancestor = first_ancestor_of_type<HTMLFormElement>();
+        if (form_ancestor)
+            set_form(form_ancestor);
+    }
+}
+
 }