--- a/dom/media/TextTrackCue.cpp
+++ b/dom/media/TextTrackCue.cpp
@@ -34,17 +34,17 @@ void
 TextTrackCue::SetDefaultCueSettings()
 {
   mPositionIsAutoKeyword = true;
   mPositionAlign = PositionAlignSetting::Center;
   mSize = 100.0;
   mPauseOnExit = false;
   mSnapToLines = true;
   mLineIsAutoKeyword = true;
-  mAlign = AlignSetting::Center;
+  mAlign = AlignSetting::Middle;
   mLineAlign = LineAlignSetting::Start;
   mVertical = DirectionSetting::_empty;
   mActive = false;
 }
 
 TextTrackCue::TextTrackCue(nsPIDOMWindowInner* aOwnerWindow,
                            double aStartTime,
                            double aEndTime,

--- a/dom/media/webvtt/vtt.jsm
+++ b/dom/media/webvtt/vtt.jsm
@@ -213,29 +213,29 @@ this.EXPORTED_SYMBOLS = ["WebVTT"];
           if (vals.length === 2) {
             settings.alt("positionAlign", vals[1], ["line-left", "center", "line-right", "auto"]);
           }
           break;
         case "size":
           settings.percent(k, v);
           break;
         case "align":
-          settings.alt(k, v, ["start", "center", "end", "left", "right"]);
+          settings.alt(k, v, ["start", "middle", "end", "left", "right"]);
           break;
         }
       }, /:/, /\s/);
 
       // Apply default values for any missing fields.
       cue.region = settings.get("region", null);
       cue.vertical = settings.get("vertical", "");
       cue.line = settings.get("line", "auto");
       cue.lineAlign = settings.get("lineAlign", "start");
       cue.snapToLines = settings.get("snapToLines", true);
       cue.size = settings.get("size", 100);
-      cue.align = settings.get("align", "center");
+      cue.align = settings.get("align", "middle");
       cue.position = settings.get("position", "auto");
       cue.positionAlign = settings.get("positionAlign", "center");
     }
 
     function skipWhitespace() {
       input = input.replace(/^\s+/, "");
     }
 
@@ -725,20 +725,21 @@ this.EXPORTED_SYMBOLS = ["WebVTT"];
       styles.writingMode = cue.vertical === "" ? "horizontal-tb"
                                                : cue.vertical === "lr" ? "vertical-lr"
                                                                        : "vertical-rl";
       styles.unicodeBidi = "plaintext";
     }
     this.applyStyles(styles, this.cueDiv);
 
     // Create an absolutely positioned div that will be used to position the cue
-    // div.
+    // div. Note, all WebVTT cue-setting alignments are equivalent to the CSS
+    // mirrors of them except "middle" which is "center" in CSS.
     this.div = window.document.createElement("div");
     styles = {
-      textAlign: cue.align,
+      textAlign: cue.align === "middle" ? "center" : cue.align,
       font: styleOptions.font,
       whiteSpace: "pre-line",
       position: "absolute"
     };
 
     if (!isIE8) {
       styles.direction = determineBidi(this.cueDiv);
       styles.writingMode = cue.vertical === "" ? "horizontal-tb"

--- a/dom/webidl/VTTCue.webidl
+++ b/dom/webidl/VTTCue.webidl
@@ -19,17 +19,17 @@ enum PositionAlignSetting {
   "line-left",
   "center",
   "line-right",
   "auto"
 };
 
 enum AlignSetting {
   "start",
-  "center",
+  "middle",
   "end",
   "left",
   "right"
 };
 
 enum DirectionSetting {
   "",
   "rl",