diff --git a/browser/base/content/test/general/browser_bug839103.js b/browser/base/content/test/general/browser_bug839103.js
index e565ba732802..217b486655d4 100644
--- a/browser/base/content/test/general/browser_bug839103.js
+++ b/browser/base/content/test/general/browser_bug839103.js
@@ -59,7 +59,7 @@ async function testBody(testRoot) {
 
   stateChanged =
     ContentTaskUtils.waitForEvent(this, "StyleSheetApplicableStateChanged", true);
-  link.disabled = true;
+  link.sheet.disabled = true;
 
   evt = await stateChanged;
   is(evt.type, "StyleSheetApplicableStateChanged", "evt.type has expected value");
diff --git a/dom/base/nsContentSink.cpp b/dom/base/nsContentSink.cpp
index 7fc184aeb6b8..ce64d5a89fa2 100644
--- a/dom/base/nsContentSink.cpp
+++ b/dom/base/nsContentSink.cpp
@@ -758,6 +758,7 @@ nsresult nsContentSink::ProcessStyleLinkFromHeader(
       aMedia,
       aAlternate ? Loader::HasAlternateRel::Yes : Loader::HasAlternateRel::No,
       Loader::IsInline::No,
+      Loader::IsExplicitlyEnabled::No,
   };
 
   auto loadResultOrErr =
diff --git a/dom/base/nsIStyleSheetLinkingElement.h b/dom/base/nsIStyleSheetLinkingElement.h
index c468d865cc83..291996f6045b 100644
--- a/dom/base/nsIStyleSheetLinkingElement.h
+++ b/dom/base/nsIStyleSheetLinkingElement.h
@@ -24,26 +24,37 @@ class nsIURI;
 
 class nsIStyleSheetLinkingElement : public nsISupports {
  public:
-  enum class ForceUpdate {
-    Yes,
+  enum class ForceUpdate : uint8_t {
     No,
+    Yes,
   };
 
-  enum class Completed {
-    Yes,
+  enum class Completed : uint8_t {
     No,
+    Yes,
   };
 
-  enum class HasAlternateRel { Yes, No };
-
-  enum class IsAlternate {
-    Yes,
+  enum class HasAlternateRel : uint8_t {
     No,
+    Yes,
   };
 
-  enum class IsInline { Yes, No };
+  enum class IsAlternate : uint8_t {
+    No,
+    Yes,
+  };
 
-  enum class MediaMatched {
+  enum class IsInline : uint8_t {
+    No,
+    Yes,
+  };
+
+  enum class IsExplicitlyEnabled : uint8_t {
+    No,
+    Yes,
+  };
+
+  enum class MediaMatched : uint8_t {
     Yes,
     No,
   };
@@ -90,14 +101,14 @@ class nsIStyleSheetLinkingElement : public nsISupports {
 
     bool mHasAlternateRel;
     bool mIsInline;
+    IsExplicitlyEnabled mIsExplicitlyEnabled;
 
     SheetInfo(const mozilla::dom::Document&, nsIContent*,
               already_AddRefed<nsIURI> aURI,
               already_AddRefed<nsIPrincipal> aTriggeringPrincipal,
-              mozilla::net::ReferrerPolicy aReferrerPolicy,
-              mozilla::CORSMode aCORSMode, const nsAString& aTitle,
-              const nsAString& aMedia, HasAlternateRel aHasAlternateRel,
-              IsInline aIsInline);
+              mozilla::net::ReferrerPolicy aReferrerPolicy, mozilla::CORSMode,
+              const nsAString& aTitle, const nsAString& aMedia, HasAlternateRel,
+              IsInline, IsExplicitlyEnabled);
 
     ~SheetInfo();
   };
diff --git a/dom/base/nsStyleLinkElement.cpp b/dom/base/nsStyleLinkElement.cpp
index 6251a75decd1..a48cbe1d69df 100644
--- a/dom/base/nsStyleLinkElement.cpp
+++ b/dom/base/nsStyleLinkElement.cpp
@@ -40,7 +40,8 @@ nsStyleLinkElement::SheetInfo::SheetInfo(
     already_AddRefed<nsIPrincipal> aTriggeringPrincipal,
     mozilla::net::ReferrerPolicy aReferrerPolicy, mozilla::CORSMode aCORSMode,
     const nsAString& aTitle, const nsAString& aMedia,
-    HasAlternateRel aHasAlternateRel, IsInline aIsInline)
+    HasAlternateRel aHasAlternateRel, IsInline aIsInline,
+    IsExplicitlyEnabled aIsExplicitlyEnabled)
     : mContent(aContent),
       mURI(aURI),
       mTriggeringPrincipal(aTriggeringPrincipal),
@@ -49,7 +50,8 @@ nsStyleLinkElement::SheetInfo::SheetInfo(
       mTitle(aTitle),
       mMedia(aMedia),
       mHasAlternateRel(aHasAlternateRel == HasAlternateRel::Yes),
-      mIsInline(aIsInline == IsInline::Yes) {
+      mIsInline(aIsInline == IsInline::Yes),
+      mIsExplicitlyEnabled(aIsExplicitlyEnabled) {
   MOZ_ASSERT(!mIsInline || aContent);
   MOZ_ASSERT_IF(aContent, aContent->OwnerDoc() == &aDocument);
 
diff --git a/dom/html/HTMLLinkElement.cpp b/dom/html/HTMLLinkElement.cpp
index 79f24ce63a9c..e01b5e79c086 100644
--- a/dom/html/HTMLLinkElement.cpp
+++ b/dom/html/HTMLLinkElement.cpp
@@ -85,12 +85,18 @@ NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(HTMLLinkElement,
 
 NS_IMPL_ELEMENT_CLONE(HTMLLinkElement)
 
-bool HTMLLinkElement::Disabled() {
+bool HTMLLinkElement::Disabled() const {
+  if (StaticPrefs::dom_link_disabled_attribute_enabled()) {
+    return GetBoolAttr(nsGkAtoms::disabled);
+  }
   StyleSheet* ss = GetSheet();
   return ss && ss->Disabled();
 }
 
-void HTMLLinkElement::SetDisabled(bool aDisabled) {
+void HTMLLinkElement::SetDisabled(bool aDisabled, ErrorResult& aRv) {
+  if (StaticPrefs::dom_link_disabled_attribute_enabled()) {
+    return SetHTMLBoolAttr(nsGkAtoms::disabled, aDisabled, aRv);
+  }
   if (StyleSheet* ss = GetSheet()) {
     ss->SetDisabled(aDisabled);
   }
@@ -314,7 +320,9 @@ nsresult HTMLLinkElement::AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
         (aName == nsGkAtoms::href || aName == nsGkAtoms::rel ||
          aName == nsGkAtoms::title || aName == nsGkAtoms::media ||
          aName == nsGkAtoms::type || aName == nsGkAtoms::as ||
-         aName == nsGkAtoms::crossorigin)) {
+         aName == nsGkAtoms::crossorigin ||
+         (aName == nsGkAtoms::disabled &&
+          StaticPrefs::dom_link_disabled_attribute_enabled()))) {
       bool dropSheet = false;
       if (aName == nsGkAtoms::rel) {
         nsAutoString value;
@@ -338,18 +346,25 @@ nsresult HTMLLinkElement::AfterSetAttr(int32_t aNameSpaceID, nsAtom* aName,
 
       const bool forceUpdate = dropSheet || aName == nsGkAtoms::title ||
                                aName == nsGkAtoms::media ||
-                               aName == nsGkAtoms::type;
+                               aName == nsGkAtoms::type ||
+                               aName == nsGkAtoms::disabled;
 
       Unused << UpdateStyleSheetInternal(
           nullptr, nullptr, forceUpdate ? ForceUpdate::Yes : ForceUpdate::No);
     }
   } else {
-    // Since removing href or rel makes us no longer link to a
-    // stylesheet, force updates for those too.
     if (aNameSpaceID == kNameSpaceID_None) {
+      if (aName == nsGkAtoms::disabled &&
+          StaticPrefs::dom_link_disabled_attribute_enabled()) {
+        mExplicitlyEnabled = true;
+      }
+      // Since removing href or rel makes us no longer link to a stylesheet,
+      // force updates for those too.
       if (aName == nsGkAtoms::href || aName == nsGkAtoms::rel ||
           aName == nsGkAtoms::title || aName == nsGkAtoms::media ||
-          aName == nsGkAtoms::type) {
+          aName == nsGkAtoms::type ||
+          (aName == nsGkAtoms::disabled &&
+           StaticPrefs::dom_link_disabled_attribute_enabled())) {
         Unused << UpdateStyleSheetInternal(nullptr, nullptr, ForceUpdate::Yes);
       }
       if ((aName == nsGkAtoms::as || aName == nsGkAtoms::type ||
@@ -415,6 +430,10 @@ Maybe<nsStyleLinkElement::SheetInfo> HTMLLinkElement::GetStyleSheetInfo() {
     return Nothing();
   }
 
+  if (StaticPrefs::dom_link_disabled_attribute_enabled() && Disabled()) {
+    return Nothing();
+  }
+
   nsAutoString title;
   nsAutoString media;
   GetTitleAndMediaForElement(*this, title, media);
@@ -444,6 +463,7 @@ Maybe<nsStyleLinkElement::SheetInfo> HTMLLinkElement::GetStyleSheetInfo() {
       media,
       alternate ? HasAlternateRel::Yes : HasAlternateRel::No,
       IsInline::No,
+      mExplicitlyEnabled ? IsExplicitlyEnabled::Yes : IsExplicitlyEnabled::No,
   });
 }
 
diff --git a/dom/html/HTMLLinkElement.h b/dom/html/HTMLLinkElement.h
index 07f6c727600f..4f15b289d0b3 100644
--- a/dom/html/HTMLLinkElement.h
+++ b/dom/html/HTMLLinkElement.h
@@ -78,8 +78,8 @@ class HTMLLinkElement final : public nsGenericHTMLElement,
   virtual bool HasDeferredDNSPrefetchRequest() override;
 
   // WebIDL
-  bool Disabled();
-  void SetDisabled(bool aDisabled);
+  bool Disabled() const;
+  void SetDisabled(bool aDisabled, ErrorResult& aRv);
 
   void GetHref(nsAString& aValue) {
     GetURIAttr(nsGkAtoms::href, nullptr, aValue);
@@ -170,8 +170,14 @@ class HTMLLinkElement final : public nsGenericHTMLElement,
   // nsStyleLinkElement
   Maybe<SheetInfo> GetStyleSheetInfo() final;
 
- protected:
   RefPtr<nsDOMTokenList> mRelList;
+
+  // The "explicitly enabled" flag. This flag is set whenever the `disabled`
+  // attribute is explicitly unset, and makes alternate stylesheets not be
+  // disabled by default anymore.
+  //
+  // See https://github.com/whatwg/html/issues/3840#issuecomment-481034206.
+  bool mExplicitlyEnabled = false;
 };
 
 }  // namespace dom
diff --git a/dom/html/HTMLStyleElement.cpp b/dom/html/HTMLStyleElement.cpp
index bfc7d5c571d5..025d529d75f1 100644
--- a/dom/html/HTMLStyleElement.cpp
+++ b/dom/html/HTMLStyleElement.cpp
@@ -45,7 +45,7 @@ NS_IMPL_ISUPPORTS_CYCLE_COLLECTION_INHERITED(HTMLStyleElement,
 
 NS_IMPL_ELEMENT_CLONE(HTMLStyleElement)
 
-bool HTMLStyleElement::Disabled() {
+bool HTMLStyleElement::Disabled() const {
   StyleSheet* ss = GetSheet();
   return ss && ss->Disabled();
 }
@@ -186,6 +186,7 @@ Maybe<nsStyleLinkElement::SheetInfo> HTMLStyleElement::GetStyleSheetInfo() {
       media,
       HasAlternateRel::No,
       IsInline::Yes,
+      IsExplicitlyEnabled::No,
   });
 }
 
diff --git a/dom/html/HTMLStyleElement.h b/dom/html/HTMLStyleElement.h
index b5066c012440..f4202000c9b6 100644
--- a/dom/html/HTMLStyleElement.h
+++ b/dom/html/HTMLStyleElement.h
@@ -56,7 +56,7 @@ class HTMLStyleElement final : public nsGenericHTMLElement,
   NS_DECL_NSIMUTATIONOBSERVER_CONTENTINSERTED
   NS_DECL_NSIMUTATIONOBSERVER_CONTENTREMOVED
 
-  bool Disabled();
+  bool Disabled() const;
   void SetDisabled(bool aDisabled);
   void GetMedia(nsAString& aValue) { GetHTMLAttr(nsGkAtoms::media, aValue); }
   void SetMedia(const nsAString& aMedia, ErrorResult& aError) {
diff --git a/dom/svg/SVGStyleElement.cpp b/dom/svg/SVGStyleElement.cpp
index 78e27134540b..f95aee2afce2 100644
--- a/dom/svg/SVGStyleElement.cpp
+++ b/dom/svg/SVGStyleElement.cpp
@@ -197,6 +197,7 @@ Maybe<nsStyleLinkElement::SheetInfo> SVGStyleElement::GetStyleSheetInfo() {
       media,
       HasAlternateRel::No,
       IsInline::Yes,
+      IsExplicitlyEnabled::No,
   });
 }
 
diff --git a/dom/webidl/HTMLLinkElement.webidl b/dom/webidl/HTMLLinkElement.webidl
index 0fc7890d6eaa..ab94dbad419e 100644
--- a/dom/webidl/HTMLLinkElement.webidl
+++ b/dom/webidl/HTMLLinkElement.webidl
@@ -14,7 +14,7 @@
 // http://www.whatwg.org/specs/web-apps/current-work/#the-link-element
 [HTMLConstructor]
 interface HTMLLinkElement : HTMLElement {
-  [Pure]
+  [CEReactions, SetterThrows, Pure]
            attribute boolean disabled;
   [CEReactions, SetterNeedsSubjectPrincipal=NonSystem, SetterThrows, Pure]
            attribute DOMString href;
diff --git a/dom/xml/XMLStylesheetProcessingInstruction.cpp b/dom/xml/XMLStylesheetProcessingInstruction.cpp
index 5c9e3d6e60d9..52093f3f5bc3 100644
--- a/dom/xml/XMLStylesheetProcessingInstruction.cpp
+++ b/dom/xml/XMLStylesheetProcessingInstruction.cpp
@@ -137,6 +137,7 @@ XMLStylesheetProcessingInstruction::GetStyleSheetInfo() {
       media,
       alternate ? HasAlternateRel::Yes : HasAlternateRel::No,
       IsInline::No,
+      IsExplicitlyEnabled::No,
   });
 }
 
diff --git a/layout/reftests/bugs/360746-1.html b/layout/reftests/bugs/360746-1.html
index 4bb7b021bea5..5e448429b3b9 100644
--- a/layout/reftests/bugs/360746-1.html
+++ b/layout/reftests/bugs/360746-1.html
@@ -4,7 +4,7 @@
     <style>
       div { height: 20px }
       #test1 {
-        background-color: red;
+        background-color: green;
       }
       #test2 {
         background-color: red;
@@ -17,7 +17,7 @@
       }
     </style>
     <link href="data:text/css;charset=utf-8," title="narrow" rel="stylesheet" type="text/css" />
-    <link href="data:text/css;charset=utf-8,%23test1%20%7Bbackground-color%3A%20green%3B%7D" title="medium" rel="alternate stylesheet" type="text/css" />
+    <link href="data:text/css;charset=utf-8,%23test1%20%7Bbackground-color%3A%20red%3B%7D" title="medium" rel="alternate stylesheet" type="text/css" />
     <link href="data:text/css;charset=utf-8,%23test2%20%7Bbackground-color%3A%20green%3B%7D" type="text/css" />    
     <link href="data:text/css;charset=utf-8,%23test3%20%7Bbackground-color%3A%20red%3B%7D" rel="stylesheet" type="text/css" />    
     <link href="data:text/css;charset=utf-8,%23test4%20%7Bbackground-color%3A%20red%3B%7D" rel="stylesheet" type="text/css" />    
diff --git a/layout/style/Loader.cpp b/layout/style/Loader.cpp
index 57d0923e1f46..2a76418730ae 100644
--- a/layout/style/Loader.cpp
+++ b/layout/style/Loader.cpp
@@ -1088,11 +1088,10 @@ static Loader::MediaMatched MediaListMatches(const MediaList* aMediaList,
  * well as setting the enabled state based on the title and whether
  * the sheet had "alternate" in its rel.
  */
-Loader::MediaMatched Loader::PrepareSheet(StyleSheet* aSheet,
-                                          const nsAString& aTitle,
-                                          const nsAString& aMediaString,
-                                          MediaList* aMediaList,
-                                          IsAlternate aIsAlternate) {
+Loader::MediaMatched Loader::PrepareSheet(
+    StyleSheet* aSheet, const nsAString& aTitle, const nsAString& aMediaString,
+    MediaList* aMediaList, IsAlternate aIsAlternate,
+    IsExplicitlyEnabled aIsExplicitlyEnabled) {
   MOZ_ASSERT(aSheet, "Must have a sheet!");
 
   RefPtr<MediaList> mediaList(aMediaList);
@@ -1106,7 +1105,8 @@ Loader::MediaMatched Loader::PrepareSheet(StyleSheet* aSheet,
   aSheet->SetMedia(mediaList);
 
   aSheet->SetTitle(aTitle);
-  aSheet->SetEnabled(aIsAlternate == IsAlternate::No);
+  aSheet->SetEnabled(aIsAlternate == IsAlternate::No ||
+                     aIsExplicitlyEnabled == IsExplicitlyEnabled::Yes);
   return MediaListMatches(mediaList, mDocument);
 }
 
@@ -1835,8 +1835,8 @@ Result<Loader::LoadSheetResult, nsresult> Loader::LoadInlineStyle(
 
   LOG(("  Sheet is alternate: %d", static_cast<int>(isAlternate)));
 
-  auto matched =
-      PrepareSheet(sheet, aInfo.mTitle, aInfo.mMedia, nullptr, isAlternate);
+  auto matched = PrepareSheet(sheet, aInfo.mTitle, aInfo.mMedia, nullptr,
+                              isAlternate, aInfo.mIsExplicitlyEnabled);
 
   InsertSheetInTree(*sheet, aInfo.mContent);
 
@@ -1940,8 +1940,8 @@ Result<Loader::LoadSheetResult, nsresult> Loader::LoadStyleLink(
 
   LOG(("  Sheet is alternate: %d", static_cast<int>(isAlternate)));
 
-  auto matched =
-      PrepareSheet(sheet, aInfo.mTitle, aInfo.mMedia, nullptr, isAlternate);
+  auto matched = PrepareSheet(sheet, aInfo.mTitle, aInfo.mMedia, nullptr,
+                              isAlternate, aInfo.mIsExplicitlyEnabled);
 
   InsertSheetInTree(*sheet, aInfo.mContent);
 
@@ -2107,7 +2107,8 @@ nsresult Loader::LoadChildSheet(StyleSheet* aParentSheet,
                      &sheet);
     NS_ENSURE_SUCCESS(rv, rv);
 
-    PrepareSheet(sheet, empty, empty, aMedia, IsAlternate::No);
+    PrepareSheet(sheet, empty, empty, aMedia, IsAlternate::No,
+                 IsExplicitlyEnabled::No);
   }
 
   MOZ_ASSERT(sheet);
@@ -2218,7 +2219,8 @@ nsresult Loader::InternalLoadNonDocumentSheet(
                    aReferrerPolicy, aIntegrity, syncLoad, state, &sheet);
   NS_ENSURE_SUCCESS(rv, rv);
 
-  PrepareSheet(sheet, empty, empty, nullptr, IsAlternate::No);
+  PrepareSheet(sheet, empty, empty, nullptr, IsAlternate::No,
+               IsExplicitlyEnabled::No);
 
   if (state == eSheetComplete) {
     LOG(("  Sheet already complete"));
diff --git a/layout/style/Loader.h b/layout/style/Loader.h
index e294fdaf8170..3c1eb3ffabaf 100644
--- a/layout/style/Loader.h
+++ b/layout/style/Loader.h
@@ -199,6 +199,7 @@ class Loader final {
   typedef nsIStyleSheetLinkingElement::HasAlternateRel HasAlternateRel;
   typedef nsIStyleSheetLinkingElement::IsAlternate IsAlternate;
   typedef nsIStyleSheetLinkingElement::IsInline IsInline;
+  typedef nsIStyleSheetLinkingElement::IsExplicitlyEnabled IsExplicitlyEnabled;
   typedef nsIStyleSheetLinkingElement::MediaMatched MediaMatched;
   typedef nsIStyleSheetLinkingElement::Update LoadSheetResult;
   typedef nsIStyleSheetLinkingElement::SheetInfo SheetInfo;
@@ -480,8 +481,8 @@ class Loader final {
   //
   // This method will set the sheet's enabled state based on aIsAlternate
   MediaMatched PrepareSheet(StyleSheet* aSheet, const nsAString& aTitle,
-                            const nsAString& aMediaString,
-                            dom::MediaList* aMediaList, IsAlternate);
+                            const nsAString& aMediaString, dom::MediaList*,
+                            IsAlternate, IsExplicitlyEnabled);
 
   // Inserts a style sheet in a document or a ShadowRoot.
   void InsertSheetInTree(StyleSheet& aSheet, nsIContent* aLinkingContent);
diff --git a/modules/libpref/init/StaticPrefList.h b/modules/libpref/init/StaticPrefList.h
index bfbbccb824d8..6943771ec35d 100644
--- a/modules/libpref/init/StaticPrefList.h
+++ b/modules/libpref/init/StaticPrefList.h
@@ -253,6 +253,18 @@ VARCACHE_PREF(
   bool, true
 )
 
+// Whether the disabled attribute in HTMLLinkElement disables the sheet loading
+// altogether, or forwards to the inner stylesheet method without attribute
+// reflection.
+//
+// Historical behavior is the second, the first is being discussed at:
+// https://github.com/whatwg/html/issues/3840
+VARCACHE_PREF(
+  "dom.link.disabled_attribute.enabled",
+   dom_link_disabled_attribute_enabled,
+  bool, true
+)
+
 VARCACHE_PREF(
   "dom.performance.enable_scheduler_timing",
   dom_performance_enable_scheduler_timing,
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.tentative.html
new file mode 100644
index 000000000000..877356f924de
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-001.tentative.html
@@ -0,0 +1,45 @@
+<!doctype html>
+<title>&lt;link disabled&gt;, HTMLLinkElement.disabled and CSSStyleSheet.disabled interactions</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+test(function() {
+  assert_equals(document.styleSheets.length, 0);
+  assert_applies(false);
+}, "<link disabled> prevents the stylesheet from being in document.styleSheets (from parser)");
+
+async_test(function(t) {
+  assert_true(link.disabled);
+
+  link.onload = t.step_func_done(function() {
+    assert_equals(document.styleSheets.length, 1);
+    let sheet = document.styleSheets[0];
+    assert_equals(sheet.ownerNode, link);
+    assert_applies(true);
+
+    link.disabled = true;
+    assert_equals(sheet.ownerNode, null);
+    assert_false(sheet.disabled);
+    assert_applies(false);
+    assert_true(link.hasAttribute("disabled"));
+
+    assert_equals(document.styleSheets.length, 0);
+    assert_applies(false);
+  });
+
+  link.disabled = false;
+  assert_true(!link.hasAttribute("disabled"));
+  assert_false(link.disabled);
+}, "HTMLLinkElement.disabled reflects the <link disabled> attribute, and behaves consistently");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.tentative.html
new file mode 100644
index 000000000000..9c46f60a6214
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-002.tentative.html
@@ -0,0 +1,38 @@
+<!doctype html>
+<title>&lt;link disabled&gt;, HTMLLinkElement.disabled and CSSStyleSheet.disabled interactions (alternate)</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="alternate stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+async_test(function(t) {
+  assert_true(link.disabled);
+
+  link.onload = t.step_func_done(function() {
+    assert_equals(document.styleSheets.length, 1);
+    let sheet = document.styleSheets[0];
+    assert_equals(sheet.ownerNode, link);
+    assert_applies(true);
+
+    link.disabled = true;
+    assert_equals(sheet.ownerNode, null);
+    assert_false(sheet.disabled);
+    assert_applies(false);
+    assert_true(link.hasAttribute("disabled"));
+    assert_equals(document.styleSheets.length, 0);
+  });
+
+  link.disabled = false;
+  assert_true(!link.hasAttribute("disabled"));
+  assert_false(link.disabled);
+}, "HTMLLinkElement.disabled reflects the <link disabled> attribute, and behaves consistently, when the sheet is an alternate");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.tentative.html
new file mode 100644
index 000000000000..fc86e6ba1f05
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-003.tentative.html
@@ -0,0 +1,32 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state persists after getting disconnected from the tree</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="alternate stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+async_test(function(t) {
+  assert_true(link.disabled);
+  link.disabled = false;
+  assert_false(link.disabled);
+  assert_true(!link.hasAttribute("disabled"));
+  link.remove();
+
+  link.onload = t.step_func_done(function() {
+    assert_equals(document.styleSheets.length, 1);
+    let sheet = document.styleSheets[0];
+    assert_equals(sheet.ownerNode, link);
+    assert_applies(true);
+  });
+
+  document.head.appendChild(link);
+}, "HTMLLinkElement.disabled's explicitly enabled state persists when disconnected and connected again");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.tentative.html
new file mode 100644
index 000000000000..7e6c34ee40d9
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-004.tentative.html
@@ -0,0 +1,40 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state doesn't persist for clones</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="alternate stylesheet" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+async_test(function(t) {
+  link.remove();
+  link.disabled = false; // `link` is explicitly enabled.
+
+  let clonesLoaded = 0;
+
+  for (let shallow of [true, false]) {
+    const clone = link.cloneNode(shallow);
+    clone.onload = t.step_func(function() {
+      assert_false(link.disabled);
+      // Even though it's not disabled, it still doesn't apply, since it's an alternate.
+      assert_applies(false);
+      if (++clonesLoaded == 2) {
+        link.onload = t.step_func_done(function() {
+          assert_false(link.disabled);
+          assert_applies(true); // `link` is still explicitly enabled.
+        });
+        document.head.appendChild(link);
+      }
+    });
+    document.head.appendChild(clone);
+  }
+}, "HTMLLinkElement.disabled's explicitly enabled state doesn't persist on clones");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.tentative.html
new file mode 100644
index 000000000000..0233f8c746ed
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-005.tentative.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" persists across rel changes</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<link title="alt" rel="yadayada" disabled href="data:text/css,html { background: green }">
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+const link = document.querySelector("link[disabled]");
+
+async_test(function(t) {
+  link.onload = t.step_func_done(function() {
+    assert_applies(true);
+    link.setAttribute("rel", "alternate stylesheet");
+    assert_applies(true);
+    assert_false(link.disabled);
+  });
+  link.disabled = false;
+  link.setAttribute("rel", "stylesheet");
+}, "HTMLLinkElement.disabled's explicitly enabled state persists regardless of rel");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.tentative.html
new file mode 100644
index 000000000000..5828e1ce1421
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-006.tentative.html
@@ -0,0 +1,26 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state isn't magically set from the setter</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+async_test(function(t) {
+  const link = document.createElement("link");
+  link.setAttribute("rel", "alternate stylesheet");
+  link.setAttribute("title", "alt");
+  link.href = "data:text/css,html { background: green }";
+  link.disabled = false; // This should do nothing, and is the point of this test.
+  link.onload = t.step_func_done(function() {
+    assert_applies(false); // Should not apply, since it's an alternate that hasn't been enabled.
+    assert_false(link.disabled);
+  });
+  document.head.appendChild(link);
+}, "HTMLLinkElement.disabled setter does nothing if the attribute isn't present already.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.tentative.html
new file mode 100644
index 000000000000..451fc3d591f2
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-007.tentative.html
@@ -0,0 +1,27 @@
+<!doctype html>
+<title>&lt;link disabled&gt;'s "explicitly enabled" state works when set explicitly back and forth</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+<script>
+function assert_applies(applies) {
+  (applies ? assert_equals : assert_not_equals)(getComputedStyle(document.documentElement).backgroundColor, "rgb(0, 128, 0)");
+}
+
+async_test(function(t) {
+  const link = document.createElement("link");
+  link.setAttribute("rel", "alternate stylesheet");
+  link.setAttribute("title", "alt");
+  link.href = "data:text/css,html { background: green }";
+  link.disabled = true;
+  link.disabled = false; // This should make it "explicitly enabled".
+  link.onload = t.step_func_done(function() {
+    assert_applies(true); // Should apply, since it's explicitly enabled.
+    assert_false(link.disabled);
+  });
+  document.head.appendChild(link);
+}, "HTMLLinkElement.disabled setter sets the explicitly enabled state if toggled back and forth.");
+</script>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html
new file mode 100644
index 000000000000..5d87bfdaf5b0
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate-ref.html
@@ -0,0 +1,7 @@
+<!DOCTYPE html>
+<title>CSS Test Reference</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<style>
+  html { background: green }
+</style>
diff --git a/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.tentative.html b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.tentative.html
new file mode 100644
index 000000000000..5b020a172b17
--- /dev/null
+++ b/testing/web-platform/tests/css/cssom/HTMLLinkElement-disabled-alternate.tentative.html
@@ -0,0 +1,15 @@
+<!DOCTYPE html>
+<html class="reftest-wait">
+<title>CSS Test: alternate stylesheets can be disabled by HTMLLinkElement.disabled if they have the disabled attribute already</title>
+<link rel="author" title="Emilio Cobos Álvarez" href="mailto:emilio@crisal.io">
+<link rel="author" title="Mozilla" href="https://mozilla.org">
+<link rel="help" href="https://bugzilla.mozilla.org/show_bug.cgi?id=1281135">
+<link rel="help" href="https://github.com/whatwg/html/issues/3840">
+<link rel="match" href="HTMLLinkElement-disabled-alternate-ref.html">
+<link title="alt" rel="alternate stylesheet" href="data:text/css,html { background: green }" disabled onload="document.documentElement.className = ''">
+<script>
+  onload = function() {
+    const link = document.querySelector("link[rel='alternate stylesheet']");
+    link.disabled = false;
+  }
+</script>