--- a/dom/media/TextTrackCue.cpp
+++ b/dom/media/TextTrackCue.cpp
@@ -28,17 +28,17 @@ NS_INTERFACE_MAP_END_INHERITING(DOMEvent
 StaticRefPtr<nsIWebVTTParserWrapper> TextTrackCue::sParserWrapper;
 
 // Set cue setting defaults based on step 19 & seq.
 // in http://dev.w3.org/html5/webvtt/#parsing
 void
 TextTrackCue::SetDefaultCueSettings()
 {
   mPosition = 50;
-  mPositionAlign = AlignSetting::Middle;
+  mPositionAlign = PositionAlignSetting::Center;
   mSize = 100;
   mPauseOnExit = false;
   mSnapToLines = true;
   mLineIsAutoKeyword = true;
   mAlign = AlignSetting::Middle;
   mLineAlign = AlignSetting::Start;
   mVertical = DirectionSetting::_empty;
 }
@@ -163,10 +163,24 @@ TextTrackCue::SetRegion(TextTrackRegion*
 {
   if (mRegion == aRegion) {
     return;
   }
   mRegion = aRegion;
   mReset = true;
 }
 
+PositionAlignSetting
+TextTrackCue::ComputedPositionAlign()
+{
+  // See spec https://w3c.github.io/webvtt/#cue-computed-position-alignment
+  if (mPositionAlign != PositionAlignSetting::Auto) {
+    return mPositionAlign;
+  } else if (mAlign == AlignSetting::Left) {
+    return PositionAlignSetting::Line_left;
+  } else if (mAlign == AlignSetting::Right) {
+    return PositionAlignSetting::Line_right;
+  }
+  return PositionAlignSetting::Center;
+}
+
 } // namespace dom
 } // namespace mozilla

--- a/dom/media/TextTrackCue.h
+++ b/dom/media/TextTrackCue.h
@@ -206,29 +206,25 @@ public:
       aRv.Throw(NS_ERROR_DOM_INDEX_SIZE_ERR);
       return;
     }
 
     mReset = true;
     mPosition = aPosition;
   }
 
-  AlignSetting PositionAlign() const
+  PositionAlignSetting PositionAlign() const
   {
     return mPositionAlign;
   }
 
-  void SetPositionAlign(AlignSetting aPositionAlign, ErrorResult& aRv)
+  void SetPositionAlign(PositionAlignSetting aPositionAlign, ErrorResult& aRv)
   {
-    if (mPositionAlign == aPositionAlign)
+    if (mPositionAlign == aPositionAlign) {
       return;
-
-    if (aPositionAlign == AlignSetting::Left ||
-        aPositionAlign == AlignSetting::Right) {
-      return aRv.Throw(NS_ERROR_DOM_SYNTAX_ERR);
     }
 
     mReset = true;
     mPositionAlign = aPositionAlign;
   }
 
   int32_t Size() const
   {
@@ -299,16 +295,18 @@ public:
     mReset = true;
   }
 
   bool HasBeenReset()
   {
     return mReset;
   }
 
+  PositionAlignSetting ComputedPositionAlign();
+
   // Helper functions for implementation.
   bool
   operator==(const TextTrackCue& rhs) const
   {
     return mId.Equals(rhs.mId);
   }
 
   const nsAString& Id() const
@@ -342,17 +340,17 @@ private:
   nsString mText;
   double mStartTime;
   double mEndTime;
 
   RefPtr<TextTrack> mTrack;
   RefPtr<HTMLTrackElement> mTrackElement;
   nsString mId;
   int32_t mPosition;
-  AlignSetting mPositionAlign;
+  PositionAlignSetting mPositionAlign;
   int32_t mSize;
   bool mPauseOnExit;
   bool mSnapToLines;
   RefPtr<TextTrackRegion> mRegion;
   DirectionSetting mVertical;
   bool mLineIsAutoKeyword;
   long mLineLong;
   AlignSetting mAlign;

--- a/dom/webidl/VTTCue.webidl
+++ b/dom/webidl/VTTCue.webidl
@@ -4,16 +4,23 @@
  * 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 PositionAlignSetting {
+  "line-left",
+  "center",
+  "line-right",
+  "auto"
+};
+
 enum AlignSetting {
   "start",
   "middle",
   "end",
   "left",
   "right"
 };
 
@@ -31,23 +38,25 @@ interface VTTCue : TextTrackCue {
   attribute DirectionSetting vertical;
   attribute boolean snapToLines;
   attribute (long or AutoKeyword) line;
   [SetterThrows]
   attribute AlignSetting lineAlign;
   [SetterThrows]
   attribute long position;
   [SetterThrows]
-  attribute AlignSetting positionAlign;
+  attribute PositionAlignSetting positionAlign;
   [SetterThrows]
   attribute long size;
   attribute AlignSetting align;
   attribute DOMString text;
   DocumentFragment getCueAsHTML();
 };
 
 // Mozilla extensions.
 partial interface VTTCue {
   [ChromeOnly]
   attribute HTMLDivElement? displayState;
   [ChromeOnly]
   readonly attribute boolean hasBeenReset;
+  [ChromeOnly]
+  readonly attribute PositionAlignSetting computedPositionAlign;
 };