<!DOCTYPE html>
<title>Reference for WebVTT rendering, default style, underline objects</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;
    font-family: sans-serif;
    font-size: 36px;
    text-align: center
}
.cue > span {
    background: rgba(0,0,0,0.8);
    color: white;
}
.cue > span > span {
    text-decoration: underline;
}
</style>
<div class="video"><span class="cue"><span>This is a <span>test subtitle</span></span></span></div>