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diff --git a/testing/web-platform/tests/common/PrefixedLocalStorage.js b/testing/web-platform/tests/common/PrefixedLocalStorage.js
new file mode 100644
index 0000000000..2f4e7b6a05
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+++ b/testing/web-platform/tests/common/PrefixedLocalStorage.js
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+/**
+ * Supports pseudo-"namespacing" localStorage for a given test
+ * by generating and using a unique prefix for keys. Why trounce on other
+ * tests' localStorage items when you can keep it "separated"?
+ *
+ * PrefixedLocalStorageTest: Instantiate in testharness.js tests to generate
+ *   a new unique-ish prefix
+ * PrefixedLocalStorageResource: Instantiate in supporting test resource
+ *   files to use/share a prefix generated by a test.
+ */
+var PrefixedLocalStorage = function () {
+  this.prefix = ''; // Prefix for localStorage keys
+  this.param = 'prefixedLocalStorage'; // Param to use in querystrings
+};
+
+PrefixedLocalStorage.prototype.clear = function () {
+  if (this.prefix === '') { return; }
+  Object.keys(localStorage).forEach(sKey => {
+    if (sKey.indexOf(this.prefix) === 0) {
+      localStorage.removeItem(sKey);
+    }
+  });
+};
+
+/**
+ * Append/replace prefix parameter and value in URI querystring
+ * Use to generate URLs to resource files that will share the prefix.
+ */
+PrefixedLocalStorage.prototype.url = function (uri) {
+  function updateUrlParameter (uri, key, value) {
+    var i         = uri.indexOf('#');
+    var hash      = (i === -1) ? '' : uri.substr(i);
+    uri           = (i === -1) ? uri : uri.substr(0, i);
+    var re        = new RegExp(`([?&])${key}=.*?(&|$)`, 'i');
+    var separator = uri.indexOf('?') !== -1 ? '&' : '?';
+    uri = (uri.match(re)) ? uri.replace(re, `$1${key}=${value}$2`) :
+      `${uri}${separator}${key}=${value}`;
+    return uri + hash;
+  }
+  return updateUrlParameter(uri, this.param, this.prefix);
+};
+
+PrefixedLocalStorage.prototype.prefixedKey = function (baseKey) {
+  return `${this.prefix}${baseKey}`;
+};
+
+PrefixedLocalStorage.prototype.setItem = function (baseKey, value) {
+  localStorage.setItem(this.prefixedKey(baseKey), value);
+};
+
+/**
+ * Listen for `storage` events pertaining to a particular key,
+ * prefixed with this object's prefix. Ignore when value is being set to null
+ * (i.e. removeItem).
+ */
+PrefixedLocalStorage.prototype.onSet = function (baseKey, fn) {
+  window.addEventListener('storage', e => {
+    var match = this.prefixedKey(baseKey);
+    if (e.newValue !== null && e.key.indexOf(match) === 0) {
+      fn.call(this, e);
+    }
+  });
+};
+
+/*****************************************************************************
+ * Use in a testharnessjs test to generate a new key prefix.
+ * async_test(t => {
+ *   var prefixedStorage = new PrefixedLocalStorageTest();
+ *   t.add_cleanup(() => prefixedStorage.cleanup());
+ *   /...
+ * });
+ */
+var PrefixedLocalStorageTest = function () {
+  PrefixedLocalStorage.call(this);
+  this.prefix = `${document.location.pathname}-${Math.random()}-${Date.now()}-`;
+};
+PrefixedLocalStorageTest.prototype = Object.create(PrefixedLocalStorage.prototype);
+PrefixedLocalStorageTest.prototype.constructor = PrefixedLocalStorageTest;
+
+/**
+ * Use in a cleanup function to clear out prefixed entries in localStorage
+ */
+PrefixedLocalStorageTest.prototype.cleanup = function () {
+  this.setItem('closeAll', 'true');
+  this.clear();
+};
+
+/*****************************************************************************
+ * Use in test resource files to share a prefix generated by a
+ * PrefixedLocalStorageTest. Will look in URL querystring for prefix.
+ * Setting `close_on_cleanup` opt truthy will make this script's window listen
+ * for storage `closeAll` event from controlling test and close itself.
+ *
+ * var PrefixedLocalStorageResource({ close_on_cleanup: true });
+ */
+var PrefixedLocalStorageResource = function (options) {
+  PrefixedLocalStorage.call(this);
+  this.options = Object.assign({}, {
+    close_on_cleanup: false
+  }, options || {});
+  // Check URL querystring for prefix to use
+  var regex = new RegExp(`[?&]${this.param}(=([^&#]*)|&|#|$)`),
+    results = regex.exec(document.location.href);
+  if (results && results[2]) {
+    this.prefix = results[2];
+  }
+  // Optionally have this window close itself when the PrefixedLocalStorageTest
+  // sets a `closeAll` item.
+  if (this.options.close_on_cleanup) {
+    this.onSet('closeAll', () => {
+      window.close();
+    });
+  }
+};
+PrefixedLocalStorageResource.prototype = Object.create(PrefixedLocalStorage.prototype);
+PrefixedLocalStorageResource.prototype.constructor = PrefixedLocalStorageResource;