title: -= operator in jQuery's css method not working in IE8
body: <p>It looks like jQuery's handling of <code>-=</code> in <code>css()</code> isn't working in IE8 for some reason. The following code doesn't through any errors, but it doesn't work either. I've narrowed it down to the operator mentioned above.</p>
  
  <pre><code>var postCount = $(".postsWrapper .window .posts article").length;
  var numberOfPages = Math.ceil(postCount / 3);
  var currentPage = 1;
  var transitioning = false;
  $(".postsWrapper button").click(function() {
      var direction = $(this).attr("data-direction");
      var postWindowWidth = $(".postsWrapper .window").width();
      if (direction == "next" &amp;&amp; currentPage &lt; numberOfPages &amp;&amp; transitioning == false) {
          transitioning = true;
          // the below line does nothing in IE8
          $(".postsWrapper .window .posts").css("marginLeft", "-=" + postWindowWidth);
          setTimeout(function() {
              transitioning = false;
          }, 1000);
          currentPage++;
      } else if (direction == "previous" &amp;&amp; currentPage &gt; 1 &amp;&amp; transitioning == false) {
          transitioning = true;
          // the below line does nothing in IE8
          $(".postsWrapper .window .posts").css("marginLeft", "+=" + postWindowWidth);
          setTimeout(function() {
              transitioning = false;
          }, 1000);
          currentPage--;
      }
  });
  </code></pre>
  
  <p>See <a href="http://www.weblinxinc.com/beta/candy-controls/demo/site/index.htm" rel="nofollow">http://www.weblinxinc.com/beta/candy-controls/demo/site/index.htm</a></p>