diff --git a/testing/web-platform/tests/webstorage/defineProperty.window.js b/testing/web-platform/tests/webstorage/defineProperty.window.js
new file mode 100644
index 000000000000..d8ab163df683
--- /dev/null
+++ b/testing/web-platform/tests/webstorage/defineProperty.window.js
@@ -0,0 +1,57 @@
+["localStorage", "sessionStorage"].forEach(function(name) {
+    [9, "x"].forEach(function(key) {
+        test(function() {
+            var desc = {
+                value: "value",
+            };
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], undefined);
+            assert_equals(storage.getItem(key), null);
+            assert_equals(Object.defineProperty(storage, key, desc), storage);
+            assert_equals(storage[key], "value");
+            assert_equals(storage.getItem(key), "value");
+        }, "Defining data property for key " + key + " on " + name);
+
+        test(function() {
+            var desc1 = {
+                value: "value",
+            };
+            var desc2 = {
+                value: "new value",
+            };
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], undefined);
+            assert_equals(storage.getItem(key), null);
+            assert_equals(Object.defineProperty(storage, key, desc1), storage);
+            assert_equals(storage[key], "value");
+            assert_equals(storage.getItem(key), "value");
+
+            assert_equals(Object.defineProperty(storage, key, desc2), storage);
+            assert_equals(storage[key], "new value");
+            assert_equals(storage.getItem(key), "new value");
+        }, "Defining data property for key " + key + " on " + name + " twice");
+
+        test(function() {
+            var desc = {
+                value: {
+                    toString: function() { return "value"; }
+                },
+            };
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], undefined);
+            assert_equals(storage.getItem(key), null);
+            assert_equals(Object.defineProperty(storage, key, desc), storage);
+            assert_equals(storage[key], "value");
+            assert_equals(storage.getItem(key), "value");
+        }, "Defining data property with toString for key " + key + " on " + name);
+    });
+});
diff --git a/testing/web-platform/tests/webstorage/set.window.js b/testing/web-platform/tests/webstorage/set.window.js
new file mode 100644
index 000000000000..479ad588910b
--- /dev/null
+++ b/testing/web-platform/tests/webstorage/set.window.js
@@ -0,0 +1,102 @@
+["localStorage", "sessionStorage"].forEach(function(name) {
+    [9, "x"].forEach(function(key) {
+        test(function() {
+            var value = "value";
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], undefined);
+            assert_equals(storage.getItem(key), null);
+            assert_equals(storage[key] = value, value);
+            assert_equals(storage[key], "value");
+            assert_equals(storage.getItem(key), "value");
+        }, "Setting property for key " + key + " on " + name);
+
+        test(function() {
+            var value = {
+                toString: function() { return "value"; }
+            };
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], undefined);
+            assert_equals(storage.getItem(key), null);
+            assert_equals(storage[key] = value, value);
+            assert_equals(storage[key], "value");
+            assert_equals(storage.getItem(key), "value");
+        }, "Setting property with toString for key " + key + " on " + name);
+
+        test(function() {
+            Storage.prototype[key] = "proto";
+            this.add_cleanup(function() { delete Storage.prototype[key]; });
+
+            var value = "value";
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], "proto");
+            assert_equals(storage.getItem(key), null);
+            assert_equals(storage[key] = value, value);
+            // Hidden because no [OverrideBuiltins].
+            assert_equals(storage[key], "proto");
+            assert_equals(Object.getOwnPropertyDescriptor(storage, key), undefined);
+            assert_equals(storage.getItem(key), "value");
+        }, "Setting property for key " + key + " on " + name + " with data property on prototype");
+
+        test(function() {
+            Storage.prototype[key] = "proto";
+            this.add_cleanup(function() { delete Storage.prototype[key]; });
+
+            var value = "value";
+
+            var storage = window[name];
+            storage.clear();
+
+            storage.setItem(key, "existing");
+
+            // Hidden because no [OverrideBuiltins].
+            assert_equals(storage[key], "proto");
+            assert_equals(Object.getOwnPropertyDescriptor(storage, key), undefined);
+            assert_equals(storage.getItem(key), "existing");
+            assert_equals(storage[key] = value, value);
+            assert_equals(storage[key], "proto");
+            assert_equals(Object.getOwnPropertyDescriptor(storage, key), undefined);
+            assert_equals(storage.getItem(key), "value");
+        }, "Setting property for key " + key + " on " + name + " with data property on prototype and existing item");
+
+        test(function() {
+            var calledSetter = [];
+            Object.defineProperty(Storage.prototype, key, {
+                "get": function() { return "proto getter"; },
+                "set": function(v) { calledSetter.push(v); },
+                configurable: true,
+            });
+            this.add_cleanup(function() { delete Storage.prototype[key]; });
+
+            var value = "value";
+
+            var storage = window[name];
+            storage.clear();
+
+            assert_equals(storage[key], "proto getter");
+            assert_equals(storage.getItem(key), null);
+            assert_equals(storage[key] = value, value);
+            // Property is hidden because no [OverrideBuiltins].
+            if (typeof key === "number") {
+                // P is an array index: call through to OrdinarySetWithOwnDescriptor()
+                assert_array_equals(calledSetter, [value]);
+                assert_equals(storage[key], "proto getter");
+                assert_equals(storage.getItem(key), null);
+            } else {
+                // P is not an array index: early return in [[Set]] step 2.
+                // https://github.com/heycam/webidl/issues/630
+                assert_equals(storage[key], "proto getter");
+                assert_equals(Object.getOwnPropertyDescriptor(storage, key), undefined);
+                assert_equals(storage.getItem(key), "value");
+            }
+        }, "Setting property for key " + key + " on " + name + " with accessor property on prototype");
+    });
+});