<!DOCTYPE html>
<title>Reference for WebVTT rendering, align:middle, position:50%</title>
<style>
html { overflow:hidden }
body { margin:0 }
.video {
    display: inline-block;
    width: 1280px;
    height: 720px;
    position: relative
}
.cue {
    position: absolute;
    bottom: 0;
    left: 0;
    right: 0;
    text-align: center;
}
.cue > span {
    font-family: Ahem, sans-serif;
    background: rgba(0,0,0,0.8);
    color: green;
    font-size: 36px
}
</style>
<div class=video><span class=cue><span>This is a test</span></span></div>