How can I create a video with subtitles on solid black stripes that extend across the entire width. I tried width, background-width...

video::cue {
    color: #ffff00;
    font-family: serif;
    font-size: 70px;
    line-height: 90px;
    width: 100%;
    background-size: 100%;
    background-image: linear-gradient(to bottom, dimgray, lightgray);
    background-position: bottom;
}
video::cue-region {
  color: #ff0000;
  width: 100%;
}