<link href="mobile.css" rel="stylesheet" media="screen and (max-width: 600px)">
But as the page loads, you're on a desktop browser where the screen is 1753px wide. The browser should just skip loading that stylesheet entirely, right? It doesn't. Thomas Steiner explains:
it turns out that the CSS spec writers and browser implementors are actually pretty darn smart about this:
The thing is, the user could always decide to resize their window (impacting width, height, aspect ratio), to print the document, etc., and even things that at first sight seem static (like the resolution) can change when a user with a multi-screen setup moves a window from say a Retina laptop screen to a bigger desktop monitor, or the user can unplug their mouse, and so on.