Upgrade nodes to custom elements on insertion, even if they're not connected to a Document

components/script/dom/element.rs

@@ -1758,10 +1758,17 @@ impl Element {
     // https://w3c.github.io/DOM-Parsing/#parsing
     pub fn parse_fragment(&self, markup: DOMString) -> Fallible<DomRoot<DocumentFragment>> {
         // Steps 1-2.
-        let context_document = document_from_node(self);
         // TODO(#11995): XML case.
         let new_children = ServoParser::parse_html_fragment(self, markup);
         // Step 3.
+        // w3c/DOM-Parsing#61
+        let context_document = {
+            if let Some(template) = self.downcast::<HTMLTemplateElement>() {
+                template.Content().upcast::<Node>().owner_doc()
+            } else {
+                document_from_node(self)
+            }
+        };
         let fragment = DocumentFragment::new(&context_document);
         // Step 4.
         for child in new_children {

components/script/dom/node.rs

@@ -1939,19 +1939,17 @@ impl Node {
                 .traverse_preorder(ShadowIncluding::Yes)
                 .filter_map(DomRoot::downcast::<Element>)
             {
-                // Step 7.7.2.
-                if descendant.is_connected() {
-                    if descendant.get_custom_element_definition().is_some() {
-                        // Step 7.7.2.1.
+                // Step 7.7.2, whatwg/dom#833
+                if descendant.get_custom_element_definition().is_some() {
+                    if descendant.is_connected() {
                         ScriptThread::enqueue_callback_reaction(
                             &*descendant,
                             CallbackReaction::Connected,
                             None,
                         );
-                    } else {
-                        // Step 7.7.2.2.
-                        try_upgrade_element(&*descendant);
                     }
+                } else {
+                    try_upgrade_element(&*descendant);
                 }
             }
         }