title: Listen for browser width for responsive web design?
body: <p>I'm trying to make my site mobile friendly.</p>
  
  <p>I want to know the browser window size so that I do something when it's narrower than 728px and something else when it's bigger.</p>
  
  <p>This must take into account resizing the window on the PC as well as changing from portrait to landscape mode in a phone.</p>
  
  <p>How can this be done?</p>