retrieve css left value with px unit

I can not open bugs.jquery.com, so there is no issue number.

# bug reproduction:
open 
http://jsfiddle.net/yiminghe/nBRgE/
in ie9 ie10 chrome firefox

# expect
alert left value with px unit in all browser
(since jquery handles % specially, then it should handle auto too.)

# actual
alert auto except in firefox