4 files changed, 31 insertions, 8 deletions
diff --git a/dom/html/HTMLLinkElement.cpp b/dom/html/HTMLLinkElement.cpp
index ed83836aac..352cbc7559 100644
--- a/dom/html/HTMLLinkElement.cpp
+++ b/dom/html/HTMLLinkElement.cpp
@@ -126,7 +126,6 @@ HTMLLinkElement::SetMozDisabled(bool aDisabled)
   return rv.StealNSResult();
 }
 
-
 NS_IMPL_STRING_ATTR(HTMLLinkElement, Charset, charset)
 NS_IMPL_URI_ATTR(HTMLLinkElement, Href, href)
 NS_IMPL_STRING_ATTR(HTMLLinkElement, Hreflang, hreflang)
@@ -407,9 +406,14 @@ HTMLLinkElement::AfterSetAttr(int32_t aNameSpaceID, nsIAtom* aName,
                                 aName == nsGkAtoms::disabled));
     }
   } else {
-    // Since removing href or rel makes us no longer link to a
-    // stylesheet, force updates for those too.
+    // If the disabled attribute is removed from a link element, the 
+    // stylesheet may be explicitly enabled.
     if (aNameSpaceID == kNameSpaceID_None) {
+      if (aName == nsGkAtoms::disabled) {
+        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 ||
@@ -508,13 +512,15 @@ HTMLLinkElement::GetStyleSheetInfo(nsAString& aTitle,
                                    nsAString& aType,
                                    nsAString& aMedia,
                                    bool* aIsScoped,
-                                   bool* aIsAlternate)
+                                   bool* aIsAlternate,
+                                   bool* aIsExplicitlyEnabled)
 {
   aTitle.Truncate();
   aType.Truncate();
   aMedia.Truncate();
   *aIsScoped = false;
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   nsAutoString rel;
   GetAttr(kNameSpaceID_None, nsGkAtoms::rel, rel);
@@ -524,10 +530,16 @@ HTMLLinkElement::GetStyleSheetInfo(nsAString& aTitle,
     return;
   }
 
+  // Is the link disabled?
   if (Disabled()) {
     return;
   }
 
+  // Is it explicitly enabled?
+  if (mExplicitlyEnabled) {
+    *aIsExplicitlyEnabled = true;
+  }
+
   nsAutoString title;
   GetAttr(kNameSpaceID_None, nsGkAtoms::title, title);
   title.CompressWhitespace();
diff --git a/dom/html/HTMLLinkElement.h b/dom/html/HTMLLinkElement.h
index e024d42bbc..190213028e 100644
--- a/dom/html/HTMLLinkElement.h
+++ b/dom/html/HTMLLinkElement.h
@@ -181,10 +181,18 @@ protected:
                                  nsAString& aType,
                                  nsAString& aMedia,
                                  bool* aIsScoped,
-                                 bool* aIsAlternate) override;
-protected:
+                                 bool* aIsAlternate,
+                                 bool* aIsExplicitlyEnabled) override;
+
   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;
+
 private:
   RefPtr<ImportLoader> mImportLoader;
 };
diff --git a/dom/html/HTMLStyleElement.cpp b/dom/html/HTMLStyleElement.cpp
index 95cf025d10..c37e1a32d7 100644
--- a/dom/html/HTMLStyleElement.cpp
+++ b/dom/html/HTMLStyleElement.cpp
@@ -223,12 +223,14 @@ HTMLStyleElement::GetStyleSheetInfo(nsAString& aTitle,
                                     nsAString& aType,
                                     nsAString& aMedia,
                                     bool* aIsScoped,
-                                    bool* aIsAlternate)
+                                    bool* aIsAlternate,
+                                    bool* aIsExplicitlyEnabled)
 {
   aTitle.Truncate();
   aType.Truncate();
   aMedia.Truncate();
   *aIsAlternate = false;
+  *aIsExplicitlyEnabled = false;
 
   nsAutoString title;
   GetAttr(kNameSpaceID_None, nsGkAtoms::title, title);
diff --git a/dom/html/HTMLStyleElement.h b/dom/html/HTMLStyleElement.h
index 8b69c3f625..7d98a494a8 100644
--- a/dom/html/HTMLStyleElement.h
+++ b/dom/html/HTMLStyleElement.h
@@ -88,7 +88,8 @@ protected:
                          nsAString& aType,
                          nsAString& aMedia,
                          bool* aIsScoped,
-                         bool* aIsAlternate) override;
+                         bool* aIsAlternate,
+                         bool* aIsExplicitlyEnabled) override;
   /**
    * Common method to call from the various mutation observer methods.
    * aContent is a content node that's either the one that changed or its