--- a/dom/media/TextTrackCue.cpp
+++ b/dom/media/TextTrackCue.cpp
@@ -34,17 +34,17 @@ TextTrackCue::SetDefaultCueSettings()
 {
   mPosition = 50;
   mPositionAlign = PositionAlignSetting::Center;
   mSize = 100;
   mPauseOnExit = false;
   mSnapToLines = true;
   mLineIsAutoKeyword = true;
   mAlign = AlignSetting::Middle;
-  mLineAlign = AlignSetting::Start;
+  mLineAlign = LineAlignSetting::Start;
   mVertical = DirectionSetting::_empty;
   mActive = false;
 }
 
 TextTrackCue::TextTrackCue(nsPIDOMWindowInner* aOwnerWindow,
                            double aStartTime,
                            double aEndTime,
                            const nsAString& aText,

--- a/dom/media/TextTrackCue.h
+++ b/dom/media/TextTrackCue.h
@@ -167,29 +167,25 @@ public:
       return;
     }
     if (aLine.IsAutoKeyword() && !mLineIsAutoKeyword) {
       mLineIsAutoKeyword = true;
       mReset = true;
     }
   }
 
-  AlignSetting LineAlign() const
+  LineAlignSetting LineAlign() const
   {
     return mLineAlign;
   }
 
-  void SetLineAlign(AlignSetting& aLineAlign, ErrorResult& aRv)
+  void SetLineAlign(LineAlignSetting& aLineAlign, ErrorResult& aRv)
   {
-    if (mLineAlign == aLineAlign)
+    if (mLineAlign == aLineAlign) {
       return;
-
-    if (aLineAlign == AlignSetting::Left ||
-        aLineAlign == AlignSetting::Right) {
-      return aRv.Throw(NS_ERROR_DOM_SYNTAX_ERR);
     }
 
     mReset = true;
     mLineAlign = aLineAlign;
   }
 
   int32_t Position() const
   {
@@ -359,17 +355,17 @@ private:
   int32_t mSize;
   bool mPauseOnExit;
   bool mSnapToLines;
   RefPtr<TextTrackRegion> mRegion;
   DirectionSetting mVertical;
   bool mLineIsAutoKeyword;
   long mLineLong;
   AlignSetting mAlign;
-  AlignSetting mLineAlign;
+  LineAlignSetting mLineAlign;
 
   // Holds the computed DOM elements that represent the parsed cue text.
   // http://www.whatwg.org/specs/web-apps/current-work/#text-track-cue-display-state
   RefPtr<nsGenericHTMLElement> mDisplayState;
   // Tells whether or not we need to recompute mDisplayState. This is set
   // anytime a property that relates to the display of the TextTrackCue is
   // changed.
   bool mReset;

--- a/dom/webidl/VTTCue.webidl
+++ b/dom/webidl/VTTCue.webidl
@@ -4,16 +4,22 @@
  * You can obtain one at http://mozilla.org/MPL/2.0/.
  *
  * The origin of this IDL file is
  * http://dev.w3.org/html5/webvtt/#the-vttcue-interface
  */
 
 enum AutoKeyword { "auto" };
 
+enum LineAlignSetting {
+  "start",
+  "center",
+  "end"
+};
+
 enum PositionAlignSetting {
   "line-left",
   "center",
   "line-right",
   "auto"
 };
 
 enum AlignSetting {
@@ -34,17 +40,17 @@ enum DirectionSetting {
  Pref="media.webvtt.enabled"]
 interface VTTCue : TextTrackCue {
   [Pref="media.webvtt.regions.enabled"]
   attribute VTTRegion? region;
   attribute DirectionSetting vertical;
   attribute boolean snapToLines;
   attribute (long or AutoKeyword) line;
   [SetterThrows]
-  attribute AlignSetting lineAlign;
+  attribute LineAlignSetting lineAlign;
   [SetterThrows]
   attribute long position;
   [SetterThrows]
   attribute PositionAlignSetting positionAlign;
   [SetterThrows]
   attribute long size;
   attribute AlignSetting align;
   attribute DOMString text;