--- a/content/media/TextTrackCue.cpp
+++ b/content/media/TextTrackCue.cpp
@@ -31,16 +31,17 @@ NS_INTERFACE_MAP_END_INHERITING(nsDOMEve
 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;
   mSize = 100;
   mPauseOnExit = false;
   mSnapToLines = true;
   mLine = WEBVTT_AUTO;
   mAlign = AlignSetting::Middle;
   mLineAlign = AlignSetting::Start;
   mVertical = DirectionSetting::_empty;
 }

--- a/content/media/TextTrackCue.h
+++ b/content/media/TextTrackCue.h
@@ -215,16 +215,36 @@ public:
       return;
     }
 
     mReset = true;
     mPosition = aPosition;
     CueChanged();
   }
 
+  AlignSetting PositionAlign() const
+  {
+    return mPositionAlign;
+  }
+
+  void SetPositionAlign(AlignSetting aPositionAlign, ErrorResult& aRv)
+  {
+    if (mPositionAlign == aPositionAlign)
+      return;
+
+    if (aPositionAlign == AlignSetting::Left ||
+        aPositionAlign == AlignSetting::Right) {
+      return aRv.Throw(NS_ERROR_DOM_SYNTAX_ERR);
+    }
+
+    mReset = true;
+    mPositionAlign = aPositionAlign;
+    CueChanged();
+  }
+
   int32_t Size() const
   {
     return mSize;
   }
 
   void SetSize(int32_t aSize, ErrorResult& aRv)
   {
     if (mSize == aSize) {
@@ -339,16 +359,17 @@ private:
   nsString mText;
   double mStartTime;
   double mEndTime;
 
   nsRefPtr<TextTrack> mTrack;
   nsRefPtr<HTMLTrackElement> mTrackElement;
   nsString mId;
   int32_t mPosition;
+  AlignSetting mPositionAlign;
   int32_t mSize;
   bool mPauseOnExit;
   bool mSnapToLines;
   nsString mRegionId;
   DirectionSetting mVertical;
   int mLine;
   AlignSetting mAlign;
   AlignSetting mLineAlign;

--- a/content/media/test/test_texttrackcue.html
+++ b/content/media/test/test_texttrackcue.html
@@ -99,16 +99,42 @@ SpecialPowers.pushPrefEnv({"set": [["med
 
       cue.lineAlign = "middle";
       is(cue.lineAlign, "middle", "Cue's line align should be middle.");
       cue.lineAlign = "START";
       is(cue.lineAlign, "middle", "Cue's line align should be middle.");
       cue.lineAlign = "end";
       is(cue.lineAlign, "end", "Cue's line align should be end.");
 
+      // Check that cue position align works properly
+      is(cue.positionAlign, "middle", "Cue's default position alignment should be middle.");
+
+      var exceptionHappened = false;
+      try {
+        cue.positionAlign = "left";
+      } catch(e) {
+        exceptionHappened = true;
+        is(e.name, "SyntaxError", "Should have thrown SyntaxError.");
+      }
+      ok(exceptionHappened, "Exception should have happened.");
+
+      exceptionHappened = false;
+      try {
+        cue.positionAlign = "right";
+      } catch(e) {
+        exceptionHappened = true;
+        is(e.name, "SyntaxError", "Should have thrown SyntaxError.");
+      }
+      ok(exceptionHappened, "Exception should have happened.");
+
+      cue.positionAlign = "start";
+      is(cue.positionAlign, "start", "Cue's position align should be start.");
+      cue.positionAlign = "end";
+      is(cue.positionAlign, "end", "Cue's position align should be end.");
+
       // Check that we can create and add new VTTCues
       var vttCue = new VTTCue(3.999, 4, "foo");
       trackElement.track.addCue(vttCue);
       is(cueList.length, 7, "Cue list length should now be 7.");
 
       // Check that new VTTCue was added correctly
       cue = cueList[6];
       is(cue.startTime, 3.999, "Cue's start time should be 3.999.");

--- a/dom/webidl/VTTCue.webidl
+++ b/dom/webidl/VTTCue.webidl
@@ -37,16 +37,18 @@ interface VTTCue : EventTarget {
   attribute boolean snapToLines;
   // XXXhumph: https://www.w3.org/Bugs/Public/show_bug.cgi?id=20651
   // attribute (long or AutoKeyword) line;
   [SetterThrows]
   attribute AlignSetting lineAlign;
   [SetterThrows]
   attribute long position;
   [SetterThrows]
+  attribute AlignSetting positionAlign;
+  [SetterThrows]
   attribute long size;
   attribute AlignSetting align;
   attribute DOMString text;
   DocumentFragment getCueAsHTML();
 
   attribute EventHandler onenter;
 
   attribute EventHandler onexit;