When an element is absolutely positioned and the value of left is decreasing the animation skips and the element jumps from the start position to the end position. However, when the value of left is increasing then the animation works as expected.

You can see this in action at http://richardwillars.com/portfolio/design.php - just watch the sheep that move from right to left..

Sample code:

$('#el').stop().animate({left: 200}, 5000);