<!DOCTYPE HTML>
<style>
div { background: yellow }
div { font-size: 12px; line-height: 1.0; width: 450px }
span { font-size: 18px }
div { text-decoration: underline overline line-through }
</style>
<!--
In a 450px container, the minimum font size at 15em per line is 30px.
This means we map 0px-45px into 30px-45px, so 12px gets mapped to 34px
and 18px gets mapped to 36px.
-->
<div>Hello <span>world</span></div>