diff --git a/packages/es-dev-server/package.json b/packages/es-dev-server/package.json
index b2f7770fb..1ad6717fc 100644
--- a/packages/es-dev-server/package.json
+++ b/packages/es-dev-server/package.json
@@ -103,6 +103,7 @@
     "@types/node-fetch": "^2.3.7",
     "@types/puppeteer": "^2.0.0",
     "@types/request": "^2.48.1",
+    "@types/whatwg-url": "^6.4.0",
     "abort-controller": "^3.0.0",
     "chai": "^4.2.0",
     "koa-proxies": "^0.8.1",
diff --git a/packages/es-dev-server/src/config.js b/packages/es-dev-server/src/config.js
index e8b826fc6..bce9f423a 100644
--- a/packages/es-dev-server/src/config.js
+++ b/packages/es-dev-server/src/config.js
@@ -192,11 +192,6 @@ export function createConfig(config) {
     openPath = basePath ? `${basePath}/` : '/';
   }
 
-  // make sure path properly starts a /
-  if (!openPath.startsWith('/')) {
-    openPath = `/${openPath}`;
-  }
-
   return {
     appIndex,
     appIndexDir,
diff --git a/packages/es-dev-server/src/start-server.js b/packages/es-dev-server/src/start-server.js
index 21f243d5f..233d0dc42 100644
--- a/packages/es-dev-server/src/start-server.js
+++ b/packages/es-dev-server/src/start-server.js
@@ -1,8 +1,17 @@
 import opn from 'opn';
 import chokidar from 'chokidar';
 import portfinder from 'portfinder';
+import { URL } from 'url';
 import { createServer } from './create-server.js';
 
+function isValidURL(str) {
+  try {
+    return !!new URL(str);
+  } catch (error) {
+    return false;
+  }
+}
+
 /** @param {import('./config.js').InternalConfig} cfg */
 export async function startServer(cfg, fileWatcher = chokidar.watch([])) {
   const result = createServer(cfg, fileWatcher);
@@ -68,7 +77,15 @@ export async function startServer(cfg, fileWatcher = chokidar.watch([])) {
       }
 
       if (cfg.openBrowser) {
-        opn(`http${cfg.http2 ? 's' : ''}://${prettyHost}:${port}${cfg.openPath}`);
+        const openPath = (() => {
+          if (isValidURL(cfg.openPath)) {
+            return cfg.openPath;
+          }
+
+          return new URL(cfg.openPath, `http${cfg.http2 ? 's' : ''}://${prettyHost}:${port}`).href;
+        })();
+
+        opn(openPath);
       }
 
       resolve();
diff --git a/yarn.lock b/yarn.lock
index ac6078474..be8d69284 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2106,16 +2106,16 @@
     standard-version "^7.0.1"
 
 "@open-wc/testing-karma-bs@file:./packages/testing-karma-bs":
-  version "1.3.4"
+  version "1.3.6"
   dependencies:
-    "@open-wc/testing-karma" "^3.2.4"
+    "@open-wc/testing-karma" "^3.2.6"
     "@types/node" "^11.13.0"
     karma-browserstack-launcher "^1.0.0"
 
 "@open-wc/testing-karma@file:./packages/testing-karma":
-  version "3.2.4"
+  version "3.2.6"
   dependencies:
-    "@open-wc/karma-esm" "^2.10.4"
+    "@open-wc/karma-esm" "^2.10.6"
     axe-core "^3.3.1"
     karma "^4.1.0"
     karma-chrome-launcher "^2.0.0"
@@ -2653,6 +2653,13 @@
   resolved "https://registry.yarnpkg.com/@types/unist/-/unist-2.0.3.tgz#9c088679876f374eb5983f150d4787aa6fb32d7e"
   integrity sha512-FvUupuM3rlRsRtCN+fDudtmytGO6iHJuuRKS1Ss0pG5z8oX0diNEw94UEL7hgDbpN94rgaK5R7sWm6RrSkZuAQ==
 
+"@types/whatwg-url@^6.4.0":
+  version "6.4.0"
+  resolved "https://registry.yarnpkg.com/@types/whatwg-url/-/whatwg-url-6.4.0.tgz#1e59b8c64bc0dbdf66d037cf8449d1c3d5270237"
+  integrity sha512-tonhlcbQ2eho09am6RHnHOgvtDfDYINd5rgxD+2YSkKENooVCFsWizJz139MQW/PV8FfClyKrNe9ZbdHrSCxGg==
+  dependencies:
+    "@types/node" "*"
+
 "@vue/babel-helper-vue-jsx-merge-props@^1.0.0":
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/@vue/babel-helper-vue-jsx-merge-props/-/babel-helper-vue-jsx-merge-props-1.0.0.tgz#048fe579958da408fb7a8b2a3ec050b50a661040"