<!DOCTYPE HTML> <style> div { background: yellow } span { background: aqua } div { font-size: 12px; width: 450px } </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. --> <div><span>Hello world</span></div>