--- a/dom/base/nsDOMAttributeMap.cpp
+++ b/dom/base/nsDOMAttributeMap.cpp
@@ -191,27 +191,42 @@ nsDOMAttributeMap::NameIsEnumerable(cons
 void
 nsDOMAttributeMap::GetSupportedNames(unsigned aFlags,
                                      nsTArray<nsString>& aNames)
 {
   if (!(aFlags & JSITER_HIDDEN)) {
     return;
   }
 
+  // For HTML elements in HTML documents, only include names that are still the
+  // same after ASCII-lowercasing, since our named getter will end up
+  // ASCII-lowercasing the given string.
+  bool lowercaseNamesOnly =
+    mContent->IsHTMLElement() && mContent->IsInHTMLDocument();
+
   const uint32_t count = mContent->GetAttrCount();
   bool seenNonAtomName = false;
   for (uint32_t i = 0; i < count; i++) {
     const nsAttrName* name = mContent->GetAttrNameAt(i);
     seenNonAtomName = seenNonAtomName || !name->IsAtom();
     nsString qualifiedName;
     name->GetQualifiedName(qualifiedName);
 
+    if (lowercaseNamesOnly &&
+        nsContentUtils::StringContainsASCIIUpper(qualifiedName)) {
+      continue;
+    }
+
+    // Omit duplicates.  We only need to do this check if we've seen a non-atom
+    // name, because that's the only way we can have two identical qualified
+    // names.
     if (seenNonAtomName && aNames.Contains(qualifiedName)) {
       continue;
     }
+
     aNames.AppendElement(qualifiedName);
   }
 }
 
 Attr*
 nsDOMAttributeMap::GetNamedItem(const nsAString& aAttrName)
 {
   bool dummy;

--- a/testing/web-platform/tests/dom/nodes/attributes.html
+++ b/testing/web-platform/tests/dom/nodes/attributes.html
@@ -581,36 +581,98 @@ test(function() {
   el.setAttribute("b", "");
   el.setAttributeNS("foo", "a", "");
   assert_array_equals(getEnumerableOwnProps1(el.attributes),
                       ["0", "1", "2"])
   assert_array_equals(getEnumerableOwnProps2(el.attributes),
                       ["0", "1", "2"])
   assert_array_equals(Object.getOwnPropertyNames(el.attributes),
                       ["0", "1", "2", "a", "b"])
+  for (var propName of Object.getOwnPropertyNames(el.attributes)) {
+    assert_true(el.attributes[propName] instanceof Attr,
+                "el.attributes has an Attr for property name " + propName);
+  }
 }, "Own property correctness with non-namespaced attribute before same-name namespaced one");
 
 test(function() {
   var el = document.createElement("div");
   el.setAttributeNS("foo", "a", "");
   el.setAttribute("b", "");
   el.setAttributeNS("", "a", "");
   assert_array_equals(getEnumerableOwnProps1(el.attributes),
                       ["0", "1", "2"])
   assert_array_equals(getEnumerableOwnProps2(el.attributes),
                       ["0", "1", "2"])
   assert_array_equals(Object.getOwnPropertyNames(el.attributes),
                       ["0", "1", "2", "a", "b"])
+  for (var propName of Object.getOwnPropertyNames(el.attributes)) {
+    assert_true(el.attributes[propName] instanceof Attr,
+                "el.attributes has an Attr for property name " + propName);
+  }
 }, "Own property correctness with namespaced attribute before same-name non-namespaced one");
 
 test(function() {
   var el = document.createElement("div");
   el.setAttributeNS("foo", "a:b", "");
   el.setAttributeNS("foo", "c:d", "");
   el.setAttributeNS("bar", "a:b", "");
   assert_array_equals(getEnumerableOwnProps1(el.attributes),
                       ["0", "1", "2"])
   assert_array_equals(getEnumerableOwnProps2(el.attributes),
                       ["0", "1", "2"])
   assert_array_equals(Object.getOwnPropertyNames(el.attributes),
                       ["0", "1", "2", "a:b", "c:d"])
+  for (var propName of Object.getOwnPropertyNames(el.attributes)) {
+    assert_true(el.attributes[propName] instanceof Attr,
+                "el.attributes has an Attr for property name " + propName);
+  }
 }, "Own property correctness with two namespaced attributes with the same name-with-prefix");
+
+test(function() {
+  var el = document.createElement("div");
+  el.setAttributeNS("foo", "A:B", "");
+  el.setAttributeNS("bar", "c:D", "");
+  el.setAttributeNS("baz", "e:F", "");
+  el.setAttributeNS("qux", "g:h", "");
+  el.setAttributeNS("", "I", "");
+  el.setAttributeNS("", "j", "");
+  assert_array_equals(Object.getOwnPropertyNames(el.attributes),
+                      ["0", "1", "2", "3", "4", "5", "g:h", "j"])
+  for (var propName of Object.getOwnPropertyNames(el.attributes)) {
+    assert_true(el.attributes[propName] instanceof Attr,
+                "el.attributes has an Attr for property name " + propName);
+  }
+}, "Own property names should only include all-lowercase qualified names for an HTML element in an HTML document");
+
+test(function() {
+  var el = document.createElementNS("", "div");
+  el.setAttributeNS("foo", "A:B", "");
+  el.setAttributeNS("bar", "c:D", "");
+  el.setAttributeNS("baz", "e:F", "");
+  el.setAttributeNS("qux", "g:h", "");
+  el.setAttributeNS("", "I", "");
+  el.setAttributeNS("", "j", "");
+  assert_array_equals(Object.getOwnPropertyNames(el.attributes),
+                      ["0", "1", "2", "3", "4", "5", "A:B", "c:D", "e:F", "g:h", "I", "j"])
+  for (var propName of Object.getOwnPropertyNames(el.attributes)) {
+    assert_true(el.attributes[propName] instanceof Attr,
+                "el.attributes has an Attr for property name " + propName);
+  }
+}, "Own property names should include all qualified names for a non-HTML element in an HTML document");
+
+test(function() {
+  var doc = document.implementation.createDocument(null, "");
+  assert_equals(doc.contentType, "application/xml");
+  var el = doc.createElementNS("http://www.w3.org/1999/xhtml", "div");
+  el.setAttributeNS("foo", "A:B", "");
+  el.setAttributeNS("bar", "c:D", "");
+  el.setAttributeNS("baz", "e:F", "");
+  el.setAttributeNS("qux", "g:h", "");
+  el.setAttributeNS("", "I", "");
+  el.setAttributeNS("", "j", "");
+  assert_array_equals(Object.getOwnPropertyNames(el.attributes),
+                      ["0", "1", "2", "3", "4", "5", "A:B", "c:D", "e:F", "g:h", "I", "j"])
+  for (var propName of Object.getOwnPropertyNames(el.attributes)) {
+    assert_true(el.attributes[propName] instanceof Attr,
+                "el.attributes has an Attr for property name " + propName);
+  }
+}, "Own property names should include all qualified names for an HTML element in a non-HTML document");
 </script>