diff --git a/Libraries/LibJS/Runtime/Realm.h b/Libraries/LibJS/Runtime/Realm.h
index 5e318c42bd2..f9c369abfec 100644
--- a/Libraries/LibJS/Runtime/Realm.h
+++ b/Libraries/LibJS/Runtime/Realm.h
@@ -30,6 +30,12 @@ public:
         virtual ~HostDefined() = default;
 
         virtual void visit_edges(Cell::Visitor&) { }
+
+        template<typename T>
+        bool fast_is() const = delete;
+
+        virtual bool is_principal_host_defined() const { return false; }
+        virtual bool is_synthetic_host_defined() const { return false; }
     };
 
     template<typename T, typename... Args>
diff --git a/Libraries/LibWeb/Bindings/Intrinsics.cpp b/Libraries/LibWeb/Bindings/Intrinsics.cpp
index 51595543122..d327dbeada4 100644
--- a/Libraries/LibWeb/Bindings/Intrinsics.cpp
+++ b/Libraries/LibWeb/Bindings/Intrinsics.cpp
@@ -31,8 +31,8 @@ bool Intrinsics::is_exposed(StringView name) const
 
 Intrinsics& host_defined_intrinsics(JS::Realm& realm)
 {
-    VERIFY(realm.host_defined());
-    return as<Bindings::HostDefined>(*realm.host_defined()).intrinsics;
+    ASSERT(realm.host_defined());
+    return static_cast<Bindings::HostDefined&>(*realm.host_defined()).intrinsics;
 }
 
 }
diff --git a/Libraries/LibWeb/Bindings/PrincipalHostDefined.h b/Libraries/LibWeb/Bindings/PrincipalHostDefined.h
index 7819d3c3380..87130056309 100644
--- a/Libraries/LibWeb/Bindings/PrincipalHostDefined.h
+++ b/Libraries/LibWeb/Bindings/PrincipalHostDefined.h
@@ -14,7 +14,7 @@
 
 namespace Web::Bindings {
 
-struct PrincipalHostDefined : public HostDefined {
+struct PrincipalHostDefined final : public HostDefined {
     PrincipalHostDefined(GC::Ref<HTML::EnvironmentSettingsObject> eso, GC::Ref<Intrinsics> intrinsics, GC::Ref<Page> page)
         : HostDefined(intrinsics)
         , environment_settings_object(eso)
@@ -23,6 +23,7 @@ struct PrincipalHostDefined : public HostDefined {
     }
     virtual ~PrincipalHostDefined() override = default;
     virtual void visit_edges(JS::Cell::Visitor& visitor) override;
+    virtual bool is_principal_host_defined() const override { return true; }
 
     GC::Ref<HTML::EnvironmentSettingsObject> environment_settings_object;
     GC::Ref<Page> page;
@@ -44,3 +45,6 @@ struct PrincipalHostDefined : public HostDefined {
 }
 
 }
+
+template<>
+inline bool JS::Realm::HostDefined::fast_is<Web::Bindings::PrincipalHostDefined>() const { return is_principal_host_defined(); }
diff --git a/Libraries/LibWeb/Bindings/SyntheticHostDefined.h b/Libraries/LibWeb/Bindings/SyntheticHostDefined.h
index ba2efb074ee..f8781a3559d 100644
--- a/Libraries/LibWeb/Bindings/SyntheticHostDefined.h
+++ b/Libraries/LibWeb/Bindings/SyntheticHostDefined.h
@@ -13,7 +13,7 @@
 
 namespace Web::Bindings {
 
-struct SyntheticHostDefined : public HostDefined {
+struct SyntheticHostDefined final : public HostDefined {
     SyntheticHostDefined(HTML::SyntheticRealmSettings synthetic_realm_settings, GC::Ref<Intrinsics> intrinsics)
         : HostDefined(intrinsics)
         , synthetic_realm_settings(move(synthetic_realm_settings))
@@ -22,8 +22,12 @@ struct SyntheticHostDefined : public HostDefined {
 
     virtual ~SyntheticHostDefined() override = default;
     virtual void visit_edges(JS::Cell::Visitor& visitor) override;
+    virtual bool is_synthetic_host_defined() const override { return true; }
 
     HTML::SyntheticRealmSettings synthetic_realm_settings;
 };
 
 }
+
+template<>
+inline bool JS::Realm::HostDefined::fast_is<Web::Bindings::SyntheticHostDefined>() const { return is_synthetic_host_defined(); }