diff --git a/Userland/Libraries/LibWeb/DOM/Document.cpp b/Userland/Libraries/LibWeb/DOM/Document.cpp
index 0848ba215ad..bb5408dba60 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.cpp
+++ b/Userland/Libraries/LibWeb/DOM/Document.cpp
@@ -288,7 +288,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Document>> Document::create_and_initialize(
     //     URL: creationURL
     //     current document readiness: "loading"
     //     about base URL: navigationParams's about base URL
-    //     FIXME: allow declarative shadow roots: true
+    //     allow declarative shadow roots: true
     auto document = HTML::HTMLDocument::create(window->realm());
     document->m_type = type;
     document->m_content_type = move(content_type);
@@ -300,6 +300,7 @@ WebIDL::ExceptionOr<JS::NonnullGCPtr<Document>> Document::create_and_initialize(
     document->set_url(*creation_url);
     document->m_readiness = HTML::DocumentReadyState::Loading;
     document->m_about_base_url = navigation_params.about_base_url;
+    document->set_allow_declarative_shadow_roots(true);
 
     document->m_window = window;
 
@@ -5209,4 +5210,16 @@ Vector<JS::Handle<DOM::Range>> Document::find_matching_text(String const& query,
     return matches;
 }
 
+// https://dom.spec.whatwg.org/#document-allow-declarative-shadow-roots
+bool Document::allow_declarative_shadow_roots() const
+{
+    return m_allow_declarative_shadow_roots;
+}
+
+// https://dom.spec.whatwg.org/#document-allow-declarative-shadow-roots
+void Document::set_allow_declarative_shadow_roots(bool allow)
+{
+    m_allow_declarative_shadow_roots = allow;
+}
+
 }
diff --git a/Userland/Libraries/LibWeb/DOM/Document.h b/Userland/Libraries/LibWeb/DOM/Document.h
index a81bd639e72..67b3b8f0345 100644
--- a/Userland/Libraries/LibWeb/DOM/Document.h
+++ b/Userland/Libraries/LibWeb/DOM/Document.h
@@ -400,6 +400,9 @@ public:
     bool is_fully_active() const;
     bool is_active() const;
 
+    [[nodiscard]] bool allow_declarative_shadow_roots() const;
+    void set_allow_declarative_shadow_roots(bool);
+
     JS::NonnullGCPtr<HTML::History> history();
     JS::NonnullGCPtr<HTML::History> history() const;
 
@@ -929,6 +932,9 @@ private:
     // instead they generate boxes as if they were siblings of the root element.
     OrderedHashTable<JS::NonnullGCPtr<Element>> m_top_layer_elements;
     OrderedHashTable<JS::NonnullGCPtr<Element>> m_top_layer_pending_removals;
+
+    // https://dom.spec.whatwg.org/#document-allow-declarative-shadow-roots
+    bool m_allow_declarative_shadow_roots { false };
 };
 
 template<>
diff --git a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
index adf52dc9199..722f6890bd2 100644
--- a/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
+++ b/Userland/Libraries/LibWeb/HTML/BrowsingContext.cpp
@@ -246,6 +246,9 @@ WebIDL::ExceptionOr<BrowsingContext::BrowsingContextAndDocument> BrowsingContext
     // about base URL: creatorBaseURL
     document->set_about_base_url(creator_base_url);
 
+    // allow declarative shadow roots: true
+    document->set_allow_declarative_shadow_roots(true);
+
     // 16. If creator is non-null, then:
     if (creator) {
         // 1. Set document's referrer to the serialization of creator's URL.