diff --git a/dom/media/TextTrackCue.cpp b/dom/media/TextTrackCue.cpp
index 2f44a0311fa1..e0a45670ac87 100644
--- a/dom/media/TextTrackCue.cpp
+++ b/dom/media/TextTrackCue.cpp
@@ -39,7 +39,7 @@ TextTrackCue::SetDefaultCueSettings()
   mSnapToLines = true;
   mLineIsAutoKeyword = true;
   mAlign = AlignSetting::Middle;
-  mLineAlign = AlignSetting::Start;
+  mLineAlign = LineAlignSetting::Start;
   mVertical = DirectionSetting::_empty;
   mActive = false;
 }
diff --git a/dom/media/TextTrackCue.h b/dom/media/TextTrackCue.h
index 24e28edd0c9d..ee716d91d885 100644
--- a/dom/media/TextTrackCue.h
+++ b/dom/media/TextTrackCue.h
@@ -172,19 +172,15 @@ public:
     }
   }
 
-  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;
@@ -364,7 +360,7 @@ private:
   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
diff --git a/dom/webidl/VTTCue.webidl b/dom/webidl/VTTCue.webidl
index 684d98fc5bc9..424c0ad900c2 100644
--- a/dom/webidl/VTTCue.webidl
+++ b/dom/webidl/VTTCue.webidl
@@ -9,6 +9,12 @@
 
 enum AutoKeyword { "auto" };
 
+enum LineAlignSetting {
+  "start",
+  "center",
+  "end"
+};
+
 enum PositionAlignSetting {
   "line-left",
   "center",
@@ -39,7 +45,7 @@ interface VTTCue : TextTrackCue {
   attribute boolean snapToLines;
   attribute (long or AutoKeyword) line;
   [SetterThrows]
-  attribute AlignSetting lineAlign;
+  attribute LineAlignSetting lineAlign;
   [SetterThrows]
   attribute long position;
   [SetterThrows]