diff --git a/polyfill.ts b/polyfill.ts
new file mode 100644
index 0000000..d697dcf
--- /dev/null
+++ b/polyfill.ts
@@ -0,0 +1,27 @@
+// Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
+// This module is browser compatible.
+
+/**
+ * @example
+ * ```ts
+ * import "https://deno.land/x/composition_key@$VERSION/polyfill.ts";
+ * import { assert } from "https://deno.land/std/testing/asserts.ts";
+ *
+ * assert(compositeKey);
+ * assert(compositeSymbol);
+ * ```
+ */
+
+// deno-lint-ignore-file no-var ban-types
+
+import { compositeKey, compositeSymbol, type Ref } from "./composite.ts";
+
+declare global {
+  var compositeKey: (
+    ...parts: [object, ...unknown[]] | [...unknown[], object]
+  ) => Ref;
+  var compositeSymbol: (...parts: readonly unknown[]) => symbol;
+}
+
+globalThis.compositeKey = compositeKey;
+globalThis.compositeSymbol = compositeSymbol;
diff --git a/polyfill_test.ts b/polyfill_test.ts
new file mode 100644
index 0000000..e2d88eb
--- /dev/null
+++ b/polyfill_test.ts
@@ -0,0 +1,20 @@
+// Copyright 2023-latest Tomoki Miyauchi. All rights reserved. MIT license.
+
+import "./polyfill.ts";
+import { assert, describe, it } from "./_dev_deps.ts";
+import {
+  compositeKey as _compositeKey,
+  compositeSymbol as _compositeSymbol,
+} from "./composite.ts";
+
+describe("compositeKey", () => {
+  it("should define global", () => {
+    assert(compositeKey === _compositeKey);
+  });
+});
+
+describe("compositeSymbol", () => {
+  it("should define global", () => {
+    assert(compositeSymbol === _compositeSymbol);
+  });
+});