unit-rem-div-width-outer.html
<style>
:root { font-size: 20px }
body { font-size: 25px }
div {
font-size: 30px;
width: 15rem;
}
#a {
border-bottom: 1px solid #f00;
}
</style>
<body>Level 1<div id=a>Level 2<div id=b>Level 3</div></div></body>
