Internet Explorer - CSS CONDITIONAL Comments

Microsoft implemented conditional comments in their browser, which allow you to link a stylesheet that will be interpreted by a browser alone.

You can also target only a certain version of IE:

While conditional comments are better, we can also target some versions of Internet Explorer using the following syntax:

.class {
  width:200px; /* All browsers */
  *width:250px; /* IE */
  _width:300px; /* IE6 */
  .width:200px; /* IE7 */
}

Since this technique is not W3C compliant,conditional comments is best option. For more details please refer Microsoft official documentation -
http://msdn.microsoft.com/en-us/library/ms537512%28VS.85%29.aspx