Use ::cue for VTT UA styles.

third_party/WebKit/Source/core/css/mediaControls.css

 /*
  * Copyright (C) 2009 Apple Inc.  All rights reserved.
  * Copyright (C) 2009 Google Inc.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 392 matching lines @@
     overflow: hidden;
     white-space: pre-wrap;
     -webkit-box-sizing: border-box;
     flex: 0 0 auto;
 }
 
 video::cue(:future) {
     color: gray;
 }
 
-video::-webkit-media-text-track-container b {
+video::cue(b) {
     font-weight: bold;
 }
 
-video::-webkit-media-text-track-container u {
+video::cue(u) {
     text-decoration: underline;
 }
 
-video::-webkit-media-text-track-container i {
+video::cue(i) {
     font-style: italic;
 }