diff --git a/dom/html/HTMLMediaElement.cpp b/dom/html/HTMLMediaElement.cpp
index bd2450dc3af4..faf5a4ec0c24 100644
--- a/dom/html/HTMLMediaElement.cpp
+++ b/dom/html/HTMLMediaElement.cpp
@@ -3129,20 +3129,20 @@ bool HTMLMediaElement::AllowedToPlay() const {
 
 uint32_t HTMLMediaElement::GetPreloadDefault() const {
   if (mMediaSource) {
-    return HTMLMediaElement::PRELOAD_ATTR_METADATA;
+    return HTMLMediaElement::PRELOAD_METADATA;
   }
   if (OnCellularConnection()) {
     return Preferences::GetInt("media.preload.default.cellular",
-                               HTMLMediaElement::PRELOAD_ATTR_NONE);
+                               HTMLMediaElement::PRELOAD_NONE);
   }
   return Preferences::GetInt("media.preload.default",
-                             HTMLMediaElement::PRELOAD_ATTR_METADATA);
+                             HTMLMediaElement::PRELOAD_METADATA);
 }
 
 uint32_t HTMLMediaElement::GetPreloadDefaultAuto() const {
   if (OnCellularConnection()) {
     return Preferences::GetInt("media.preload.auto.cellular",
-                               HTMLMediaElement::PRELOAD_ATTR_METADATA);
+                               HTMLMediaElement::PRELOAD_METADATA);
   }
   return Preferences::GetInt("media.preload.auto",
                              HTMLMediaElement::PRELOAD_ENOUGH);
@@ -3167,14 +3167,13 @@ void HTMLMediaElement::UpdatePreloadAction() {
       // media.preload.default pref, or just preload metadata if not present.
       nextAction = static_cast<PreloadAction>(preloadDefault);
     } else if (val->Type() == nsAttrValue::eEnum) {
-      PreloadAttrValue attr =
-          static_cast<PreloadAttrValue>(val->GetEnumValue());
-      if (attr == HTMLMediaElement::PRELOAD_ATTR_EMPTY ||
-          attr == HTMLMediaElement::PRELOAD_ATTR_AUTO) {
+      MediaPreloadAttrValue attr =
+          static_cast<MediaPreloadAttrValue>(val->GetEnumValue());
+      if (attr == MediaPreloadAttrValue::PRELOAD_ATTR_AUTO) {
         nextAction = static_cast<PreloadAction>(preloadAuto);
-      } else if (attr == HTMLMediaElement::PRELOAD_ATTR_METADATA) {
+      } else if (attr == MediaPreloadAttrValue::PRELOAD_ATTR_METADATA) {
         nextAction = HTMLMediaElement::PRELOAD_METADATA;
-      } else if (attr == HTMLMediaElement::PRELOAD_ATTR_NONE) {
+      } else if (attr == MediaPreloadAttrValue::PRELOAD_ATTR_NONE) {
         nextAction = HTMLMediaElement::PRELOAD_NONE;
       }
     } else {
@@ -4946,21 +4945,16 @@ bool HTMLMediaElement::ParseAttribute(int32_t aNamespaceID, nsAtom* aAttribute,
                                       const nsAString& aValue,
                                       nsIPrincipal* aMaybeScriptedPrincipal,
                                       nsAttrValue& aResult) {
-  // Mappings from 'preload' attribute strings to an enumeration.
-  static const nsAttrValue::EnumTableEntry kPreloadTable[] = {
-      {"", HTMLMediaElement::PRELOAD_ATTR_EMPTY},
-      {"none", HTMLMediaElement::PRELOAD_ATTR_NONE},
-      {"metadata", HTMLMediaElement::PRELOAD_ATTR_METADATA},
-      {"auto", HTMLMediaElement::PRELOAD_ATTR_AUTO},
-  };
-
   if (aNamespaceID == kNameSpaceID_None) {
     if (aAttribute == nsGkAtoms::crossorigin) {
       ParseCORSValue(aValue, aResult);
       return true;
     }
     if (aAttribute == nsGkAtoms::preload) {
-      return aResult.ParseEnumValue(aValue, kPreloadTable, false);
+      return aResult.ParseEnumValue(aValue, kPreloadTable, false,
+                                    // The default value is "auto" if aValue is
+                                    // not a recognised value.
+                                    kPreloadDefaultType);
     }
   }
 
diff --git a/dom/html/HTMLMediaElement.h b/dom/html/HTMLMediaElement.h
index 4eab5d4ea5fd..6e151457977c 100644
--- a/dom/html/HTMLMediaElement.h
+++ b/dom/html/HTMLMediaElement.h
@@ -110,6 +110,25 @@ enum class StreamCaptureBehavior : uint8_t {
   FINISH_WHEN_ENDED
 };
 
+/**
+ * Possible values of the 'preload' attribute.
+ */
+enum MediaPreloadAttrValue : uint8_t {
+  PRELOAD_ATTR_NONE,      // set to "none"
+  PRELOAD_ATTR_METADATA,  // set to "metadata"
+  PRELOAD_ATTR_AUTO       // set to "auto"
+};
+
+// Mappings from 'preload' attribute strings to an enumeration.
+static const nsAttrValue::EnumTableEntry kPreloadTable[] = {
+    {"none", MediaPreloadAttrValue::PRELOAD_ATTR_NONE},
+    {"metadata", MediaPreloadAttrValue::PRELOAD_ATTR_METADATA},
+    {"auto", MediaPreloadAttrValue::PRELOAD_ATTR_AUTO},
+};
+
+static constexpr const nsAttrValue::EnumTableEntry* kPreloadDefaultType =
+    &kPreloadTable[std::size(kPreloadTable) - 1];
+
 class HTMLMediaElement : public nsGenericHTMLElement,
                          public MediaDecoderOwner,
                          public PrincipalChangeObserver<MediaStreamTrack>,
@@ -521,7 +540,7 @@ class HTMLMediaElement : public nsGenericHTMLElement,
       nsGkAtoms::none->ToString(aValue);
       return;
     }
-    GetEnumAttr(nsGkAtoms::preload, nullptr, aValue);
+    GetEnumAttr(nsGkAtoms::preload, kPreloadDefaultType->tag, aValue);
   }
   void SetPreload(const nsAString& aValue, ErrorResult& aRv) {
     if (mSrcAttrStream) {
@@ -1165,16 +1184,6 @@ class HTMLMediaElement : public nsGenericHTMLElement,
    */
   void NotifyShutdownEvent();
 
-  /**
-   * Possible values of the 'preload' attribute.
-   */
-  enum PreloadAttrValue : uint8_t {
-    PRELOAD_ATTR_EMPTY,     // set to ""
-    PRELOAD_ATTR_NONE,      // set to "none"
-    PRELOAD_ATTR_METADATA,  // set to "metadata"
-    PRELOAD_ATTR_AUTO       // set to "auto"
-  };
-
   /**
    * The preloading action to perform. These dictate how we react to the
    * preload attribute. See mPreloadAction.
diff --git a/dom/html/test/mochitest.toml b/dom/html/test/mochitest.toml
index c973a61af200..aab2a1cb1b9c 100644
--- a/dom/html/test/mochitest.toml
+++ b/dom/html/test/mochitest.toml
@@ -660,8 +660,6 @@ skip-if = [
 
 ["test_bug666200.html"]
 
-["test_bug666666.html"]
-
 ["test_bug669012.html"]
 
 ["test_bug674558.html"]
diff --git a/dom/html/test/test_bug666666.html b/dom/html/test/test_bug666666.html
deleted file mode 100644
index a3c22d4e0f2b..000000000000
--- a/dom/html/test/test_bug666666.html
+++ /dev/null
@@ -1,32 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=666666
--->
-<head>
-  <title>Test for Bug 666666</title>
-  <script src="/tests/SimpleTest/SimpleTest.js"></script>
-  <script type="application/javascript" src="reflect.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=666666">Mozilla Bug 666666</a>
-<p id="display"></p>
-<div id="content" style="display: none">
-  
-</div>
-<pre id="test">
-<script type="application/javascript">
-/** Test for Bug 666666 **/
-["audio", "video"].forEach(function(element) {
-  reflectLimitedEnumerated({
-    element: document.createElement(element),
-    attribute: "preload",
-    validValues: ["none", "metadata", "auto"],
-    invalidValues: ["cheesecake", ""]
-  });
-});
-</script>
-</pre>
-</body>
-</html>
diff --git a/dom/media/test/mochitest_compat.toml b/dom/media/test/mochitest_compat.toml
index 7f34caafb9f9..40ddb129db7c 100644
--- a/dom/media/test/mochitest_compat.toml
+++ b/dom/media/test/mochitest_compat.toml
@@ -957,8 +957,6 @@ skip-if = ["appname == 'seamonkey'"] #  Seamonkey: Bug 598252, bug 1307337, bug
 
 ["test_preload_actions.html"]
 
-["test_preload_attribute.html"]
-
 ["test_preload_suspend.html"]
 
 ["test_preserve_playbackrate_after_ui_play.html"]
diff --git a/dom/media/test/test_preload_attribute.html b/dom/media/test/test_preload_attribute.html
deleted file mode 100644
index 1e415035c554..000000000000
--- a/dom/media/test/test_preload_attribute.html
+++ /dev/null
@@ -1,44 +0,0 @@
-<!DOCTYPE HTML>
-<html>
-<!--
-https://bugzilla.mozilla.org/show_bug.cgi?id=479863
--->
-<head>
-  <title>Test for Bug 479863</title>
-  <script src="/tests/SimpleTest/SimpleTest.js"></script>
-  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
-</head>
-<body>
-<a target="_blank" href="https://bugzilla.mozilla.org/show_bug.cgi?id=479863">Mozilla Bug 479863</a>
-<p id="display"></p>
-<div id="content" style="display: none">
-  
-</div>
-
-<video id='v1'></video><audio id='a1'></audio>
-<video id='v2' preload="auto"></video><audio id='a2' preload="auto"></audio>
-
-<pre id="test">
-<script type="application/javascript">
-var v1 = document.getElementById('v1');
-var a1 = document.getElementById('a1');
-var v2 = document.getElementById('v2');
-var a2 = document.getElementById('a2');
-is(v1.getAttribute("preload"), null, "video preload via getAttribute should be null by default");
-is(a1.getAttribute("preload"), null, "video preload via getAttribute should be null by default");
-is(v1.preload, "", "v1.preload should be empty by default");
-is(a1.preload, "", "a1.preload should be empty by default");
-is(v2.preload, "auto", "v2.preload should be auto");
-is(a2.preload, "auto", "a2.preload should be auto");
-
-v1.preload = "auto";
-a1.preload = "auto";
-is(v1.preload, "auto", "video.preload should be auto");
-is(a1.preload, "auto", "audio.preload should be auto");
-is(v1.getAttribute("preload"), "auto", "video preload attribute should be auto");
-is(a1.getAttribute("preload"), "auto", "video preload attribute should be auto");
-
-</script>
-</pre>
-</body>
-</html>