#a, #b {
 width  : 200px;
 border : 1px solid red;
}
#a {
 margin : 10px auto 10px auto;      
}
#b {
 float       : left;
 margin    : -1px 0 0 -202px;          
}

<div id="a">A

<div id="b">B</div>

</div>

function getStyle(el, style){
if(!document.getElementById) return;
 
var value = el.style[style];
 
if(!value)
if(document.defaultView)
value = document.defaultView.
getComputedStyle(el, "").getPropertyValue(style);
 
else if(el.currentStyle)
value = el.currentStyle[style];
 
return value;
}
 
function toCamelCase(input) {
var ret = input;
while(ret.match(/(-[a-zA-Z])/) != null) {
ret = ret.replace(ret[1], ret[1].toUpperCase().substring(1));
}
 
alert(getStyle('a', 'margin-left'));