diff --git a/src/common/mainWorldContentScriptBridge/Client.js b/src/common/mainWorldContentScriptBridge/Client.js
new file mode 100644
index 0000000..70e567b
--- /dev/null
+++ b/src/common/mainWorldContentScriptBridge/Client.js
@@ -0,0 +1,50 @@
+export const kDefaultTimeout = 10 * 1000;  // 10 seconds
+
+export default class MainWorldContentScriptBridgeClient {
+  constructor(CSTarget, MWTarget, timeout) {
+    if (!CSTarget || !MWTarget) {
+      throw new Error(
+          `[MWOptionsWatcherClient] CSTarget and MWTarget are compulsory.`);
+    }
+
+    this.CSTarget = CSTarget;
+    this.MWTarget = MWTarget;
+    this.timeout = timeout ?? kDefaultTimeout;
+  }
+
+  _sendRequestWithoutCallback(action, request, uuid) {
+    if (!uuid) uuid = self.crypto.randomUUID();
+    const data = {
+      target: this.CSTarget,
+      uuid,
+      action,
+      request,
+    };
+    window.postMessage(data, '*');
+  }
+
+  _sendRequest(action, request) {
+    return new Promise((res, rej) => {
+      const uuid = self.crypto.randomUUID();
+
+      let timeoutId;
+      let listener = e => {
+        if (e.source !== window || e.data?.target !== this.MWTarget ||
+            e.data?.uuid !== uuid)
+          return;
+
+        window.removeEventListener('message', listener);
+        clearTimeout(timeoutId);
+        res(e.data?.response);
+      };
+      window.addEventListener('message', listener);
+
+      timeoutId = setTimeout(() => {
+        window.removeEventListener('message', listener);
+        rej(new Error('Timed out while waiting response.'));
+      }, this.timeout);
+
+      this._sendRequestWithoutCallback(action, request, uuid);
+    });
+  }
+}
diff --git a/src/common/mainWorldContentScriptBridge/Server.js b/src/common/mainWorldContentScriptBridge/Server.js
new file mode 100644
index 0000000..321a068
--- /dev/null
+++ b/src/common/mainWorldContentScriptBridge/Server.js
@@ -0,0 +1,37 @@
+export default class MainWorldContentScriptBridgeServer {
+  constructor(CSTarget, MWTarget) {
+    if (!CSTarget || !MWTarget) {
+      throw new Error(
+          `[MWOptionsWatcherServer] CSTarget and MWTarget are compulsory.`);
+    }
+
+    this.CSTarget = CSTarget;
+    this.MWTarget = MWTarget;
+    this.handler = () => {};
+  }
+
+  // Handler should be an action of the form (uuid, action, request) => {...}.
+  setUpHandler(handler) {
+    this.handler = handler;
+    window.addEventListener('message', e => this.#handleMessage(e));
+  }
+
+  #handleMessage(e) {
+    const uuid = e.data?.uuid;
+    if (e.source !== window || e.data?.target !== this.CSTarget || !uuid)
+      return;
+
+    const action = e.data?.action;
+    const request = e.data?.request;
+    return this.handler(uuid, action, request);
+  }
+
+  _respond(uuid, response) {
+    const data = {
+      target: this.MWTarget,
+      uuid,
+      response,
+    };
+    window.postMessage(data, window.origin);
+  }
+}