what they don’t understand. Separating structure and behaviour isn’t quite so easy. If you give a browser some JavaScript that it doesn’t understand, not only will it not apply the desired behaviour, it will refuse to parse the rest of the JavaScript.</p>

<p>Before you use a particular feature in JavaScript, it’s worth testing to see if that feature is supported. This kind of feature detection can save your website’s visitors from having a broken experience because of an unsupported feature. If 