diff --git a/custom-elements/pseudo-class-defined.html b/custom-elements/pseudo-class-defined.html
index 60d88cffb517c0..24cb5fe4cd3922 100644
--- a/custom-elements/pseudo-class-defined.html
+++ b/custom-elements/pseudo-class-defined.html
@@ -90,4 +90,22 @@
     assert_equals(style.color, expected ? defined : not_defined, 'getComputedStyle');
   }, `${description} should ${expected ? 'be' : 'not be'} :defined`);
 }
+
+test(function () {
+    var log = [];
+    var instance = document.createElement('my-custom-element-2');
+    document.body.appendChild(instance);
+    customElements.define('my-custom-element-2',class extends HTMLElement {
+        constructor() {
+            super();
+            log.push([this, 'begin']);
+            assert_false(this.matches(":defined"), "During construction, this should not match :defined");
+            log.push([this, 'end']);
+        }
+    });
+    assert_equals(log.length, 2);
+    assert_array_equals(log[0], [instance, 'begin']);
+    assert_array_equals(log[1], [instance, 'end']);
+}, 'this.matches(:defined) should not match during an upgrade');
+
 </script>
diff --git a/custom-elements/upgrading.html b/custom-elements/upgrading.html
index d6b8ed387540ae..ac0fdff7a21115 100644
--- a/custom-elements/upgrading.html
+++ b/custom-elements/upgrading.html
@@ -12,6 +12,9 @@
 <script src="resources/custom-elements-helpers.js"></script>
 </head>
 <body>
+<infinite-cloning-element-1></infinite-cloning-element-1>
+<infinite-cloning-element-2 id="a"></infinite-cloning-element-2>
+<infinite-cloning-element-2 id="b"></infinite-cloning-element-2>
 <div id="log"></div>
 <script>
 setup({allow_uncaught_exception:true});
@@ -203,6 +206,53 @@
 }, 'If definition\'s disable shadow is true and element\'s shadow root is ' +
     'non-null, then throw a "NotSupportedError" DOMException.');
 
+test(() => {
+    var log = [];
+
+    customElements.define('infinite-cloning-element-1',class extends HTMLElement {
+        constructor() {
+            super();
+            log.push([this, 'begin']);
+            // Potential infinite recursion:
+            customElements.upgrade(this);
+            log.push([this, 'end']);
+        }
+    });
+
+    assert_equals(log.length, 2);
+    const instance = document.querySelector("infinite-cloning-element-1");
+    assert_array_equals(log[0], [instance, 'begin']);
+    assert_array_equals(log[1], [instance, 'end']);
+}, 'Infinite constructor recursion with upgrade(this) should not be possible');
+
+test(() => {
+    var log = [];
+
+    customElements.define('infinite-cloning-element-2',class extends HTMLElement {
+        constructor() {
+            super();
+            log.push([this, 'begin']);
+            const b = document.querySelector("#b");
+            b.remove();
+            // While this constructor is running for "a", "b" is still
+            // undefined, and so inserting it into the document will enqueue a
+            // second upgrade reaction for "b" in addition to the one enqueued
+            // by defining x-foo.
+            document.body.appendChild(b);
+            log.push([this, 'end']);
+        }
+    });
+
+    assert_equals(log.length, 4);
+    const instanceA = document.querySelector("#a");
+    const instanceB = document.querySelector("#b");
+    assert_array_equals(log[0], [instanceA, 'begin']);
+    assert_array_equals(log[1], [instanceB, 'begin']);
+    assert_array_equals(log[2], [instanceB, 'end']);
+    assert_array_equals(log[3], [instanceA, 'end']);
+}, 'Infinite constructor recursion with appendChild should not be possible');
+
+
 </script>
 </body>
 </html>
