diff --git a/.github/workflows/steps.yml b/.github/workflows/steps.yml
index 80da76c..b7126d0 100644
--- a/.github/workflows/steps.yml
+++ b/.github/workflows/steps.yml
@@ -15,7 +15,7 @@ jobs:
 
       - run: npm ci
 
-      - run: npm test
+      - run: npm run test:ci
         env:
           BROWSER_STACK_USERNAME: ${{ secrets.BROWSER_STACK_USERNAME }}
           BROWSER_STACK_ACCESS_KEY: ${{ secrets. BROWSER_STACK_ACCESS_KEY }}
diff --git a/browserstack-karma.js b/browserstack-karma.js
index 63dcd01..bf395bb 100644
--- a/browserstack-karma.js
+++ b/browserstack-karma.js
@@ -90,6 +90,7 @@ module.exports = {
     browser_version: "11.0",
     os: "Windows",
     os_version: "7",
+    es5: true,
   },
   BS_IE_10: {
     base: "BrowserStack",
@@ -97,5 +98,6 @@ module.exports = {
     browser_version: "10.0",
     os: "Windows",
     os_version: "7",
+    es5: true,
   },
 };
diff --git a/karma.conf.js b/karma.conf.js
index 1090e4e..571791c 100644
--- a/karma.conf.js
+++ b/karma.conf.js
@@ -1,6 +1,7 @@
 const ci = !!process.env.CI;
 const watch = !!process.env.WATCH;
 const live = !!process.env.LIVE;
+const es5 = !!process.env.ES5;
 
 const ip = "bs-local.com";
 
@@ -9,8 +10,11 @@ const browserstack = require("./browserstack-karma.js");
 // https://www.browserstack.com/open-source (text search "parallels")
 const BROWSERSTACK_OPEN_SOURCE_CONCURRENCY = 5;
 
+const getBrowserstackBrowsers = () =>
+  Object.keys(browserstack).filter((k) => !!browserstack[k].es5 === es5);
+
 const browsers = ci
-  ? Object.keys(browserstack)
+  ? getBrowserstackBrowsers()
   : live
   ? undefined
   : watch
@@ -43,6 +47,12 @@ module.exports = function (config) {
       compilerOptions: {
         ...require("./tsconfig.json").compilerOptions,
         ...require("./test/tsconfig.json").compilerOptions,
+        sourceMap: false,
+        inlineSourceMap: true,
+        target: es5 ? "es5" : "es6",
+      },
+      bundlerOptions: {
+        sourceMap: true,
       },
       include: process.env.FILES_PATTERN.split(",").concat("src/**/*.ts"),
     },
diff --git a/package.json b/package.json
index bd44e22..b6ab904 100644
--- a/package.json
+++ b/package.json
@@ -33,6 +33,7 @@
     "unit": "cross-env FILES_PATTERN=\"test/unit/*.ts,test/unit/*.tsx\" karma start karma.conf.js",
     "benchmark": "cross-env FILES_PATTERN=\"test-bundles/benchmark/**/*.js\" karma start karma.conf.cjs --concurrency=1",
     "release": "release-it",
+    "test:ci": "npm test && cross-env ES5=true npm run unit",
     "test": "npm run build && npm run lint && npm run unit"
   },
   "devDependencies": {
diff --git a/test/unit/core.ts b/test/unit/core.ts
index 0b84930..3208ce1 100644
--- a/test/unit/core.ts
+++ b/test/unit/core.ts
@@ -41,15 +41,7 @@ function map(fn: any, list: any[]) {
 
 const inner = prop("innerHTML");
 
-class A extends HTMLParagraphElement {}
-class B extends HTMLParagraphElement {}
-
 describe("snabbdom", function () {
-  before(function () {
-    customElements.define("p-a", A, { extends: "p" });
-    customElements.define("p-b", B, { extends: "p" });
-  });
-
   let elm: any, vnode0: any;
   beforeEach(function () {
     elm = document.createElement("div");
@@ -199,13 +191,11 @@ describe("snabbdom", function () {
     it("handles classes from both selector and property", function () {
       elm = patch(vnode0, h("div", [h("i.has", { class: { classes: true } })]))
         .elm;
-      assert(elm.firstChild.classList.contains("has"));
-      assert(elm.firstChild.classList.contains("classes"));
-    });
-    it("can create custom elements", function () {
-      const vnode1 = h("p", { is: "p-a" });
-      elm = patch(vnode0, vnode1).elm;
-      assert(elm instanceof A);
+      assert(elm.firstChild.classList.contains("has"), "has `has` class");
+      assert(
+        elm.firstChild.classList.contains("classes"),
+        "has `classes` class"
+      );
     });
     it("can create elements with text content", function () {
       elm = patch(vnode0, h("div", ["I am a string"])).elm;
@@ -378,14 +368,52 @@ describe("snabbdom", function () {
       patch(vnode1, vnode2);
       assert.strictEqual((elm as any).a, "foo");
     });
-    it("handles changing is attribute", function () {
-      const vnode1 = h("p", { is: "p-a" });
-      const vnode2 = h("p", { is: "p-b" });
+    describe("custom elements", function () {
+      if ("customElements" in window) {
+        describe("customized built-in element", function () {
+          const isSafari = /^((?!chrome|android).)*safari/i.test(
+            navigator.userAgent
+          );
 
-      elm = patch(vnode0, vnode1).elm;
-      assert(elm instanceof A);
-      elm = patch(vnode1, vnode2).elm;
-      assert(elm instanceof B);
+          if (!isSafari) {
+            class A extends HTMLParagraphElement {}
+            class B extends HTMLParagraphElement {}
+
+            before(function () {
+              if ("customElements" in window) {
+                customElements.define("p-a", A, { extends: "p" });
+                customElements.define("p-b", B, { extends: "p" });
+              }
+            });
+            it("can create custom elements", function () {
+              if ("customElements" in window) {
+                const vnode1 = h("p", { is: "p-a" });
+                elm = patch(vnode0, vnode1).elm;
+                assert(elm instanceof A);
+              } else {
+                this.skip();
+              }
+            });
+            it("handles changing is attribute", function () {
+              const vnode1 = h("p", { is: "p-a" });
+              const vnode2 = h("p", { is: "p-b" });
+
+              elm = patch(vnode0, vnode1).elm;
+              assert(elm instanceof A);
+              elm = patch(vnode1, vnode2).elm;
+              assert(elm instanceof B);
+            });
+          } else {
+            it.skip("safari does not support customized built-in elements", () => {
+              assert(false);
+            });
+          }
+        });
+      } else {
+        it.skip("browser does not support custom elements", () => {
+          assert(false);
+        });
+      }
     });
     describe("using toVNode()", function () {
       it("can remove previous children of the root element", function () {