diff --git a/src/browser/polyfill/polyfill.zig b/src/browser/polyfill/polyfill.zig
index c14c75a2..bf6f9227 100644
--- a/src/browser/polyfill/polyfill.zig
+++ b/src/browser/polyfill/polyfill.zig
@@ -61,8 +61,11 @@ pub const Loader = struct {
 
                 .{ "litNonce", {} },
                 .{ "litHtmlVersions", {} },
+                .{ "litElementVersions", {} },
                 .{ "litHtmlPolyfillSupport", {} },
                 .{ "litElementHydrateSupport", {} },
+                .{ "litElementPolyfillSupport", {} },
+                .{ "reactiveElementVersions", {} },
 
                 .{ "recaptcha", {} },
                 .{ "grecaptcha", {} },
diff --git a/src/browser/tests/custom_elements/registry.html b/src/browser/tests/custom_elements/registry.html
index 8ead6ae2..064aa9f9 100644
--- a/src/browser/tests/custom_elements/registry.html
+++ b/src/browser/tests/custom_elements/registry.html
@@ -81,3 +81,41 @@
     testing.expectEqual('NO-HYPHEN-INVALID', el.tagName);
 }
 </script>
+
+<script id="whenDefined_already_defined">
+{
+    class AlreadyDefined extends HTMLElement {}
+    customElements.define('already-defined', AlreadyDefined);
+
+    const promise = customElements.whenDefined('already-defined');
+    testing.expectEqual('object', typeof promise);
+    testing.expectEqual(true, promise instanceof Promise);
+}
+</script>
+
+<script id="whenDefined_not_yet_defined">
+{
+    const promise = customElements.whenDefined('future-element');
+    testing.expectEqual('object', typeof promise);
+    testing.expectEqual(true, promise instanceof Promise);
+
+    // Now define it
+    class FutureElement extends HTMLElement {}
+    customElements.define('future-element', FutureElement);
+}
+</script>
+
+<script id="whenDefined_same_promise">
+{
+    const promise1 = customElements.whenDefined('pending-element');
+    const promise2 = customElements.whenDefined('pending-element');
+
+    // Should return the same promise for the same name
+    testing.expectEqual(true, promise1 === promise2);
+
+    // Define it so cleanup happens
+    class PendingElement extends HTMLElement {}
+    customElements.define('pending-element', PendingElement);
+}
+</script>
+
diff --git a/src/browser/webapi/CustomElementRegistry.zig b/src/browser/webapi/CustomElementRegistry.zig
index 2f8a912c..727e74ed 100644
--- a/src/browser/webapi/CustomElementRegistry.zig
+++ b/src/browser/webapi/CustomElementRegistry.zig
@@ -30,6 +30,7 @@ const CustomElementDefinition = @import("CustomElementDefinition.zig");
 const CustomElementRegistry = @This();
 
 _definitions: std.StringHashMapUnmanaged(*CustomElementDefinition) = .{},
+_when_defined: std.StringHashMapUnmanaged(js.PersistentPromiseResolver) = .{},
 
 const DefineOptions = struct {
     extends: ?[]const u8 = null,
@@ -103,6 +104,10 @@ pub fn define(self: *CustomElementRegistry, name: []const u8, constructor: js.Fu
 
         _ = page._undefined_custom_elements.swapRemove(idx);
     }
+
+    if (self._when_defined.fetchRemove(name)) |entry| {
+        try entry.value.resolve(constructor);
+    }
 }
 
 pub fn get(self: *CustomElementRegistry, name: []const u8) ?js.Function {
@@ -114,6 +119,25 @@ pub fn upgrade(self: *CustomElementRegistry, root: *Node, page: *Page) !void {
     try upgradeNode(self, root, page);
 }
 
+pub fn whenDefined(self: *CustomElementRegistry, name: []const u8, page: *Page) !js.Promise {
+    if (self._definitions.get(name)) |definition| {
+        return page.js.resolvePromise(definition.constructor);
+    }
+
+    const gop = try self._when_defined.getOrPut(page.arena, name);
+    if (gop.found_existing) {
+        return gop.value_ptr.promise();
+    }
+    errdefer _ = self._when_defined.remove(name);
+    const owned_name = try page.dupeString(name);
+
+    const resolver = try page.js.createPromiseResolver(.page);
+    gop.key_ptr.* = owned_name;
+    gop.value_ptr.* = resolver;
+
+    return resolver.promise();
+}
+
 fn upgradeNode(self: *CustomElementRegistry, node: *Node, page: *Page) !void {
     if (node.is(Element)) |element| {
         try upgradeElement(self, element, page);
@@ -222,6 +246,7 @@ pub const JsApi = struct {
     pub const define = bridge.function(CustomElementRegistry.define, .{ .dom_exception = true });
     pub const get = bridge.function(CustomElementRegistry.get, .{ .null_as_undefined = true });
     pub const upgrade = bridge.function(CustomElementRegistry.upgrade, .{});
+    pub const whenDefined = bridge.function(CustomElementRegistry.whenDefined, .{});
 };
 
 const testing = @import("../../testing.zig");