javascript - One-pixel error in offsetLeft (Firefox) -

assume have simple structure this:

<div class="a">   <div class="b"></div> </div> 

i want give both #a , #b fixed width , height , center #b in #a:

#a { width: 199px, height: 199px; position: relative; } #b { width: 100px; height: 100px; position: absolute;      top: 0; bottom: 0; left: 0; right: 0; margin: auto } 

the trick here cannot centered because difference 99px , cannot divided evenly, there single-pixel shift somewhere. problem screen shows me 1 thing, javascript reports another. if read offsetleft of #b, see 50px, when measure on screenshot, it's 49px. (offsettop seems ok, although in real app seem one-pixel error it).

here's jsfiddle.

this happens firefox 47.0.