Add support for declarative shadow DOM

html5ever/src/tree_builder/mod.rs

@@ -1376,8 +1376,31 @@ where
     fn insert_phantom(&self, name: LocalName) -> Handle {
         self.insert_element(Push, ns!(html), name, vec![])
     }
+
+    /// <https://html.spec.whatwg.org/multipage/parsing.html#insert-an-element-at-the-adjusted-insertion-location>
+    fn insert_foreign_element(
+        &self,
+        tag: Tag,
+        ns: Namespace,
+        only_add_to_element_stack: bool,
+    ) -> Handle {
+        let adjusted_insertion_location = self.appropriate_place_for_insertion(None);
+        let qname = QualName::new(None, ns, tag.name);
+        let elem = create_element(&self.sink, qname.clone(), tag.attrs.clone());
+
+        if !only_add_to_element_stack {
+            self.insert_at(adjusted_insertion_location, AppendNode(elem.clone()));
+        }
+
+        self.push(&elem);
+
+        elem
+    }
     //§ END
 
+    /// <https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhead>
+    ///
+    /// A start tag whose tag name is "template"
     fn should_attach_declarative_shadow(&self, tag: &Tag) -> bool {
         let adjusted_insertion_location = self.appropriate_place_for_insertion(None);
 
@@ -1392,7 +1415,7 @@ where
         // template start tag's shadowrootmode is not in the none state
         let is_shadow_root_mode = tag.attrs.iter().any(|attr| {
             attr.name.local == local_name!("shadowrootmode")
-                && (attr.value.to_string() == *"open" || attr.value.to_string() == *"close")
+                && (attr.value.as_ref() == "open" || attr.value.as_ref() == "closed")
         });
 
         // Check if intended_parent's document allows declarative shadow roots
@@ -1416,9 +1439,17 @@ where
         is_shadow_root_mode && allow_declarative_shadow_roots && adjusted_current_node_not_topmost
     }
 
-    fn attach_declarative_shadow(&self, tag: &Tag) -> Result<(), String> {
+    /// <https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhead>
+    ///
+    /// A start tag whose tag name is "template"
+    fn attach_declarative_shadow(
+        &self,
+        tag: &Tag,
+        shadow_host: &Handle,
+        template: &Handle,
+    ) -> Result<(), String> {
         self.sink
-            .attach_declarative_shadow(self.open_elems.borrow().last().unwrap(), tag.attrs.clone())
+            .attach_declarative_shadow(shadow_host, template, tag.attrs.clone())
     }
 
     fn create_formatting_element_for(&self, tag: Tag) -> Handle {

html5ever/src/tree_builder/rules.rs

@@ -160,10 +160,24 @@ where
                     self.template_modes.borrow_mut().push(InTemplate);
 
                     if (self.should_attach_declarative_shadow(&tag)) {
-                        if self.attach_declarative_shadow(&tag).is_err() {
-                            // TODO:
-                            // insert at the adjusted insertion location
-                            // with the result of insert a foreign element for template tag
+                        // Attach shadow path
+
+                        // Step 1. Let declarative shadow host element be adjusted current node.
+                        let mut shadow_host = self.open_elems.borrow().last().unwrap().clone();
+                        if self.is_fragment() && self.open_elems.borrow().len() == 1 {
+                            shadow_host = self.context_elem.borrow().clone().unwrap();
+                        }
+
+                        // Step 2. Let template be the result of insert a foreign element for template start tag, with HTML namespace and true.
+                        let template = self.insert_foreign_element(tag.clone(), ns!(html), true);
+
+                        // Step 3 - 8.
+                        // Attach a shadow root with declarative shadow host element, mode, clonable, serializable, delegatesFocus, and "named".
+                        if self.attach_declarative_shadow(&tag, &shadow_host, &template).is_err() {
+                            // Step 8.1.1. Insert an element at the adjusted insertion location with template.
+                            // Pop the current template element created in step 2 first.
+                            self.pop();
+                            self.insert_element_for(tag);
                         }
                     } else {
                         self.insert_element_for(tag);

markup5ever/interface/tree_builder.rs

@@ -269,6 +269,7 @@ pub trait TreeSink {
     fn attach_declarative_shadow(
         &self,
         _location: &Self::Handle,
+        _template: &Self::Handle,
         _attrs: Vec<Attribute>,
     ) -> Result<(), String> {
         Err(String::from(