Due to how the DOM structure of WebVTT is set up, the font of the track container ends up imposing a minimum line height on the ::cue.
Minimal example is:
<!doctype html>
<style>
.container {
font: 5vh sans-serif;
}
.cue {
font: 10px sans-serif;
color: white;
background-color: rgba(0, 0, 0, .8);
}
</style>
<div class="container">
<span class="cue">This is<br>some cue<br>text</span>
</div>
This doesn't seem great? Maybe the container line-height should default to 0?