e “stuff”—can be added and updated by the hour, the minute, or even the second. In between are the layers of structure, presentation, and behaviour: <abbr>HTML</abbr>, <abbr>CSS</abbr>, and JavaScript.</p>

<p>Those layers can be loosely‐coupled, but they aren’t completely independent. Just as a building cannot have furniture without first having rooms and walls, a style sheet needs some markup to act upon. The coupling between structure and presentation is handled through selectors in <abbr>CSS<