Retrieving percentage CSS values (in firefox)

Question

I have a problem retrieving the exact css property value (in '%') on firefox.

Suppose we have this extremely simple markup:

<div id="box">box</div>

and this css:

#box{
    width:200px;
    height:200px;
    left:10%;
    position:absolute;
    background:red;
}

and I'd like to retrieve the left position (in '%') by js

It's obv very easy with mootools (demo -> http://jsfiddle.net/steweb/AWdzB/):

var left = $('box').getStyle('left');

or jQuery (demo -> http://jsfiddle.net/steweb/RaVyU/):

var left = $('#box').css('left');

or by plain js (demo -> http://jsfiddle.net/steweb/tUAKA/):

function getStyle(el,styleProp){ //from ppk's quirksmode
    var x = document.getElementById(el);
    if (x.currentStyle)
        var y = x.currentStyle[styleProp];
    else if (window.getComputedStyle)
        var y = document.defaultView.getComputedStyle(x,null).getPropertyValue(styleProp);
    return y;
}

var left = getStyle('box','left');

But if you try it on firefox (8.0.1) you'll see that the result is not correct (should be 10%, but it's 91px). The questions are: is there a bug on this newer version of firefox? Does anyone knows if it's a known bug? Am I doing something wrong?

Thanks :)

Update: I tried it also on older firefox releases, and it's not correct (it always returns px value).. for completeness, it works correctly on IE

Answer 1

The correct answer is a comment on the bug I filed on bugzilla

https://bugzilla.mozilla.org/show_bug.cgi?id=707691#c7

To get the correct % value (on firefox too) the element's (or one of its parents) display should be set to none

Test : http://jsfiddle.net/4RKsM/

The unclear thing is: why on the same browser/version (see, firefox 7 on XP/win7 or Opera 11.5 on mac osx / ubuntu) but on different os, the behav is different?

Btw, the spec @thg435 posted (and reported on mdn) is still in flux.