diff --git a/html5ever/src/tree_builder/mod.rs b/html5ever/src/tree_builder/mod.rs
index 88afa725..60e594bc 100644
--- a/html5ever/src/tree_builder/mod.rs
+++ b/html5ever/src/tree_builder/mod.rs
@@ -1378,6 +1378,47 @@ where
     }
     //§ END
 
+    fn should_attach_declarative_shadow(&self, tag: &Tag) -> bool {
+        let adjusted_insertion_location = self.appropriate_place_for_insertion(None);
+
+        let (intended_parent, _node2) = match adjusted_insertion_location {
+            LastChild(ref p) | BeforeSibling(ref p) => (p.clone(), None),
+            TableFosterParenting {
+                ref element,
+                ref prev_element,
+            } => (element.clone(), Some(prev_element.clone())),
+        };
+
+        // 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() == String::from("open") || attr.value.to_string() == String::from("close"))
+        });
+
+        // Check if intended_parent's document allows declarative shadow roots
+        let allow_declarative_shadow_roots = self.sink.allow_declarative_shadow_roots(&intended_parent);
+
+        // the adjusted current node is not the topmost element in the stack of open elements
+        let adjusted_current_node_not_topmost = match self.open_elems.borrow().first() {
+            // The stack grows downwards; the topmost node on the stack is the first one added to the stack
+            // The current node is the bottommost node in this stack of open elements.
+            //
+            // (1) The adjusted current node is the context element if the parser was created as part of the HTML fragment parsing algorithm
+            // and the stack of open elements has only one element in it (fragment case);
+            // (2) otherwise, the adjusted current node is the current node (the bottomost node)
+            //
+            // => adjusted current node != topmost element in the stack when the stack size > 1
+            Some(_) => self.open_elems.borrow().len() > 1,
+            None => true,
+        };
+
+        return is_shadow_root_mode && allow_declarative_shadow_roots && adjusted_current_node_not_topmost;
+    }
+
+    fn attach_declarative_shadow(&self, tag: &Tag) -> Result<(), String> {
+        self.sink.attach_declarative_shadow(self.open_elems.borrow().last().unwrap(), tag.attrs.clone())
+    }
+
     fn create_formatting_element_for(&self, tag: Tag) -> Handle {
         // FIXME: This really wants unit tests.
         let mut first_match = None;
diff --git a/html5ever/src/tree_builder/rules.rs b/html5ever/src/tree_builder/rules.rs
index eec6faf7..870e0ce3 100644
--- a/html5ever/src/tree_builder/rules.rs
+++ b/html5ever/src/tree_builder/rules.rs
@@ -105,6 +105,7 @@ where
             }),
 
             //§ parsing-main-inhead
+            // https://html.spec.whatwg.org/multipage/parsing.html#parsing-main-inhead
             InHead => match_token!(token {
                 CharacterTokens(NotSplit, text) => SplitWhitespace(text),
                 CharacterTokens(Whitespace, text) => self.append_text(text),
@@ -153,11 +154,21 @@ where
                 </body> </html> </br> => else,
 
                 tag @ <template> => {
-                    self.insert_element_for(tag);
                     self.active_formatting.borrow_mut().push(Marker);
                     self.frameset_ok.set(false);
                     self.mode.set(InTemplate);
                     self.template_modes.borrow_mut().push(InTemplate);
+
+                    if (self.should_attach_declarative_shadow(&tag)) {
+                        if let Err(_) = self.attach_declarative_shadow(&tag) {
+                            // TODO:
+                            // insert at the adjusted insertion location
+                            // with the result of insert a foreign element for template tag
+                        }
+                    } else {
+                        self.insert_element_for(tag);
+                    }
+
                     Done
                 }
 
diff --git a/markup5ever/interface/tree_builder.rs b/markup5ever/interface/tree_builder.rs
index f1a30a8e..6e5e7787 100644
--- a/markup5ever/interface/tree_builder.rs
+++ b/markup5ever/interface/tree_builder.rs
@@ -260,6 +260,19 @@ pub trait TreeSink {
     fn complete_script(&self, _node: &Self::Handle) -> NextParserState {
         NextParserState::Continue
     }
+
+    fn allow_declarative_shadow_roots(&self, _intended_parent: &Self::Handle) -> bool {
+        return true;
+    }
+
+    /// Attach declarative shadow
+    fn attach_declarative_shadow(
+        &self,
+        _location: &Self::Handle,
+        _attrs: Vec<Attribute>,
+    ) -> Result<(), String> {
+        Err(String::from("No implementation for attach_declarative_shadow"))
+    }
 }
 
 /// Trace hooks for a garbage-collected DOM.
diff --git a/markup5ever/local_names.txt b/markup5ever/local_names.txt
index 1aaee007..04bd2e27 100644
--- a/markup5ever/local_names.txt
+++ b/markup5ever/local_names.txt
@@ -849,6 +849,10 @@ separator
 separators
 set
 setdiff
+shadowrootclonable
+shadowrootdelegatesfocus
+shadowrootmode
+shadowrootserializable
 shape
 shape-rendering
 show