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+<!DOCTYPE HTML>
+<title>Tests of some tricky semantics around NamedNodeMap and the element.attributes collection</title>
+<link rel="author" title="Domenic Denicola" href="mailto:d@domenic.me">
+<link rel="help" href="https://dom.spec.whatwg.org/#interface-namednodemap">
+<link rel="help" href="https://dom.spec.whatwg.org/#dom-element-attributes">
+
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+
+<script>
+"use strict";
+
+test(() => {
+
+  const element = document.createElement("div");
+  element.setAttribute("x", "first");
+
+  assert_equals(element.attributes.length, 1, "one attribute");
+  assert_equals(element.attributes.x.value, "first");
+
+}, "an attribute set by setAttribute should be accessible as a field on the `attributes` field of an Element");
+
+test(() => {
+
+  const element = document.createElement("div");
+  const map = element.attributes;
+
+  assert_equals(map.length, 0);
+
+  const attr1 = document.createAttribute("attr1");
+  map.setNamedItem(attr1);
+  assert_equals(map.attr1, attr1);
+  assert_equals(map.length, 1);
+
+  const attr2 = document.createAttribute("attr2");
+  map.setNamedItem(attr2);
+  assert_equals(map.attr2, attr2);
+  assert_equals(map.length, 2);
+
+  const rm1 = map.removeNamedItem("attr1");
+  assert_equals(rm1, attr1);
+  assert_equals(map.length, 1);
+
+  const rm2 = map.removeNamedItem("attr2");
+  assert_equals(rm2, attr2);
+  assert_equals(map.length, 0);
+
+}, "setNamedItem and removeNamedItem on `attributes` should add and remove fields from `attributes`");
+
+test(() => {
+
+  const element = document.createElement("div");
+  const map = element.attributes;
+
+  const fooAttribute = document.createAttribute("foo");
+  map.setNamedItem(fooAttribute);
+
+  const itemAttribute = document.createAttribute("item");
+  map.setNamedItem(itemAttribute);
+
+  assert_equals(map.foo, fooAttribute);
+  assert_equals(map.item, NamedNodeMap.prototype.item);
+  assert_equals(typeof map.item, "function");
+
+  map.removeNamedItem("item");
+  assert_equals(map.item, NamedNodeMap.prototype.item);
+  assert_equals(typeof map.item, "function");
+
+}, "setNamedItem and removeNamedItem on `attributes` should not interfere with existing method names");
+
+test(() => {
+
+  const element = document.createElement("div");
+  element.setAttributeNS(null, "x", "first");
+
+  assert_equals(element.attributes.length, 1, "one attribute");
+  assert_equals(element.attributes.x.value, "first");
+
+}, "an attribute with a null namespace should be accessible as a field on the `attributes` field of an Element");
+
+test(() => {
+
+  const element = document.createElement("div");
+  element.setAttributeNS("foo", "x", "first");
+
+  assert_equals(element.attributes.length, 1, "one attribute");
+  assert_equals(element.attributes.x.value, "first");
+
+}, "an attribute with a set namespace should be accessible as a field on the `attributes` field of an Element");
+
+test(() => {
+
+  const element = document.createElement("div");
+  element.setAttributeNS("foo", "setNamedItem", "first");
+
+  assert_equals(element.attributes.length, 1, "one attribute");
+  assert_equals(typeof element.attributes.setNamedItem, "function");
+
+}, "setting an attribute should not overwrite the methods of an `NamedNodeMap` object");
+
+test(() => {
+
+  const element = document.createElement("div");
+  element.setAttributeNS("foo", "toString", "first");
+
+  assert_equals(element.attributes.length, 1, "one attribute");
+  assert_equals(typeof element.attributes.toString, "function");
+
+}, "setting an attribute should not overwrite the methods defined by prototype ancestors of an `NamedNodeMap` object");
+
+test(() => {
+
+  const element = document.createElement("div");
+  element.setAttributeNS("foo", "length", "first");
+
+  assert_equals(element.attributes.length, 1, "one attribute");
+
+}, "setting an attribute should not overwrite the the length property of an `NamedNodeMap` object");
+
+</script>