<!DOCTYPE html>
<title>Reference for WebVTT rendering, line:50% should be vertically centered</title>
<style>
html { overflow:hidden }
body { margin:0 }
.video {
    display: inline-block;
    width: 1280px;
    height: 720px;
    position: relative
}
.cue {
    position: absolute;
    top: 50%;
    left: 0;
    right: 0;
    margin-top: -18px;
    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 subtitle</span></span></div>