diff --git a/dom/media/TextTrackCue.cpp b/dom/media/TextTrackCue.cpp
index 1a441a0aeb17..823d49863952 100644
--- a/dom/media/TextTrackCue.cpp
+++ b/dom/media/TextTrackCue.cpp
@@ -9,6 +9,7 @@
 #include "mozilla/dom/TextTrackRegion.h"
 #include "nsComponentManagerUtils.h"
 #include "mozilla/ClearOnShutdown.h"
+#include "unicode/ubidi.h"
 
 extern mozilla::LazyLogModule gTextTrackLog;
 
@@ -205,10 +206,24 @@ PositionAlignSetting TextTrackCue::ComputedPositionAlign() {
     return PositionAlignSetting::Line_left;
   } else if (mAlign == AlignSetting::Right) {
     return PositionAlignSetting::Line_right;
+  } else if (mAlign == AlignSetting::Start) {
+    return IsTextBaseDirectionLTR() ? PositionAlignSetting::Line_left
+                                    : PositionAlignSetting::Line_right;
+  } else if (mAlign == AlignSetting::End) {
+    return IsTextBaseDirectionLTR() ? PositionAlignSetting::Line_right
+                                    : PositionAlignSetting::Line_left;
   }
   return PositionAlignSetting::Center;
 }
 
+bool TextTrackCue::IsTextBaseDirectionLTR() const {
+  // The returned result by `ubidi_getBaseDirection` might be `neutral` if the
+  // text only contains netural charaters. In this case, we would treat its
+  // base direction as LTR.
+  return ubidi_getBaseDirection(mText.BeginReading(), mText.Length()) !=
+         UBIDI_RTL;
+}
+
 void TextTrackCue::NotifyDisplayStatesChanged() {
   if (!mReset) {
     return;
diff --git a/dom/media/TextTrackCue.h b/dom/media/TextTrackCue.h
index e2d3e556b146..9569a5a2358d 100644
--- a/dom/media/TextTrackCue.h
+++ b/dom/media/TextTrackCue.h
@@ -298,6 +298,8 @@ class TextTrackCue final : public DOMEventTargetHelper {
   void SetDefaultCueSettings();
   nsresult StashDocument();
 
+  bool IsTextBaseDirectionLTR() const;
+
   RefPtr<Document> mDocument;
   nsString mText;
   double mStartTime;