title: Changing width using jQuery .css() and em
body: <p>I need to change the width of an div using jQuery.</p>
  
  <p>When I use the following, it works:</p>
  
  <pre><code>$('#footer').css('width', '100%');
  $('#footer').css('width', '-=239');
  </code></pre>
  
  <p>When I use this, it works:</p>
  
  <pre><code>$('#footer').css('width', '100%');
  $('#footer').css('width', '-=239px');
  </code></pre>
  
  <p>But when I use this it doesn't do anything:</p>
  
  <pre><code>$('#footer').css('width', '100%');
  $('#footer').css('width', '-=21em');
  </code></pre>
  
  <p>Is there a way to make jQuery work with <code>em</code>? or to calculate <code>em</code> to <code>px</code> and set a variable for example and subtract that value?</p>
  
  <p>edit: thanks everybody for correcting my spelling and code!</p>