--- a/dom/media/test/test_texttrackcue.html
+++ b/dom/media/test/test_texttrackcue.html
@@ -139,17 +139,17 @@ SpecialPowers.pushPrefEnv({"set": [["med
       cue.lineAlign = "center";
       is(cue.lineAlign, "center", "Cue's line align should be center.");
       cue.lineAlign = "START";
       is(cue.lineAlign, "center", "Cue's line align should be center.");
       cue.lineAlign = "end";
       is(cue.lineAlign, "end", "Cue's line align should be end.");
 
       // Check that cue position align works properly
-      is(cue.positionAlign, "center", "Cue's default position alignment should be center.");
+      is(cue.positionAlign, "auto", "Cue's default position alignment should be auto.");
 
       cue.positionAlign = "line-left";
       is(cue.positionAlign, "line-left", "Cue's position align should be line-left.");
       cue.positionAlign = "auto";
       is(cue.positionAlign, "auto", "Cue's position align should be auto.");
       cue.positionAlign = "line-right";
       is(cue.positionAlign, "line-right", "Cue's position align should be line-right.");
 

--- a/dom/media/test/test_webvtt_positionalign.html
+++ b/dom/media/test/test_webvtt_positionalign.html
@@ -38,17 +38,17 @@ function runTest() {
   isnot(window.TextTrackCue, undefined, "TextTrackCue should be defined.");
   isnot(window.VTTCue, undefined, "VTTCue should be defined.");
 
   info("--- check the type of first parsed cue ---");
   ok(cues[0] instanceof TextTrackCue, "Cue should be an instanceof TextTrackCue.");
   ok(cues[0] instanceof VTTCue, "Cue should be an instanceof VTTCue.");
 
   info("--- check the cue's position alignment ---");
-  is(cues[0].positionAlign, "center",     cues[0].text);
+  is(cues[0].positionAlign, "auto",     cues[0].text);
   is(cues[1].positionAlign, "line-left",  cues[1].text);
   is(cues[2].positionAlign, "center",     cues[2].text);
   is(cues[3].positionAlign, "line-right", cues[3].text);
   is(cues[4].positionAlign, "auto",       cues[4].text);
 
   info("--- check the cue's computed position alignment ---");
   // The "computedPositionAlign" is the chrome-only attributes, we need to get
   // the chrome privilege for cues.

--- a/dom/media/test/vttPositionAlign.vtt
+++ b/dom/media/test/vttPositionAlign.vtt
@@ -1,12 +1,12 @@
 WEBVTT
 
 00:00.000 --> 00:00.500
-Cue 0 : PositionAlign should be "center".
+Cue 0 : PositionAlign should be "auto".
 
 00:00.700 --> 00:00.800 position:50%,line-left
 Cue 1 : PositionAlign should be "line-left".
 
 00:00.700 --> 00:00.800 position:50%,center
 Cue 2 : PositionAlign should be "center".
 
 00:00.700 --> 00:00.800 position:50%,line-right