IE returns values for all properties (not just font-size) in whatever units were specified in the stylesheet (em, in, cm, etc...).

I've been working on a hack/fix that boils down to:

• insert a temporary element into the dom as a sibling of the target element
• set the temp element's font-size to match the target element's font-size
• set the temp element's width to the non-px value
• measure the temp element using offsetWidth (which is always in pixels)

Here's an example:

(function($){

var tmp = $('<span style="border: 0; display: block; padding: 0; position: absolute; overflow: hidden;" />')[0];
	
$.px = function(elem, val) {
    if(elem && elem.parentNode && /(cm|em|in|pt)$/.test(val)) {
    var style = (document.defaultView && document.defaultView.getComputedStyle && document.defaultView.getComputedStyle(elem, null)) || elem.currentStyle;
    tmp.style.fontSize = (style.getPropertyValue && style.getPropertyValue('font-size')) || style.fontSize;
    tmp.style.width = val;
    elem.parentNode.insertBefore(tmp, elem);
    val = tmp.offsetWidth + 'px';
    tmp.parentNode.removeChild(tmp);
    return val;
};

})(jQuery);	

Which can be used like so:

var elem = $('p')[0];
var value = $(elem).css('font-size');
// px returns undefined if the value can't be converted, so default to the original value.
var fixed = $.px(elem, value) || value;

Fixed in SVN rev [3132].

It seems that the bug reappeared in 1.2.1 release.

Yes, I am seeing this using the latest release of jQuery (1.2.1).

var fontSize = $('body').css('font-size');

In IE it's returning 795px.

My CSS on the body is

font: 68.75%/1.366 Verdana, Tahoma, Arial, Helvetica, sans-serif;

actually i've noticed

.css('font-size')

works different than

.css('fontSize')

which returns the same cross-browser...just not the computed style. This works fine for my application at the moment...

Hmm. I'm not seeing any difference between .css('fontSize') and .css('font-size') using 1.2.6.

Looks like the problem now might have to do with the value being computed up the DOM tree. When font size is set to a percentage, it's affected by the window width.

See example here (jQuery should say "18px" for all of them):

http://test.learningjquery.com/font-size.html

Doing a bit of testing reveals the discrepancy has an approximated formula for the returned ie value (based on a em value from css):

n^2 x 12 = ie value in pixels.

so

css: 1.0em, firefox: 12px, ie:12px

css: 2.0em, firefox: 24px, ie:48px

css: 3.0em, firefox: 36px, ie:108px

to fetch an approximated px value in ie:

square root of (the iepx value / 12)

so

parseint(Math.sqrt($(obj).css("font-size")/12),10);

n^2 x 12 = ie value in pixels.

hmm the page didn't like my carat. it's n(to the power of)2 x 12

Replying to [comment:11 binarypusher]:

to fetch an approximated px value in ie: parseint(Math.sqrt($(obj).css("font-size")/12),10);

should have checked this before hand. sorry for the multiposts ... can't see how to edit an entry. here's the working version:

(Math.sqrt(parseInt($(this).css("font-size"),10).toFixed(2)/12)*12).toFixed(1);

In IE, when font-size is set to "2.0em" in CSS, you get '2.0 * 2.0 em' size in pixels with css("font-size").

Because the size is calculated "2.0em" relational size of "the element's font size (..is 2.0em)".

And when font-size is set to "80%" is CSS, css("font-size") is calculated "80%" relational size of "the element's width".

The patch (jquery-css-fontsize-ie.patch) will fix it.

after patched, curCSS method always calculates "1em" of "the element's font size" when css("font-size") called.

The fix by haruka orks well. I´m using a similiar approach to fix this odd behaviour in my clip-Animation-Plugin (http://plugins.jquery.com/files/clip.js.txt). But I would suggest, that you use a 100em base instead of 1em, to get information about "subpixels". getComputedStyle also returns floating point pixel-values and its important, if you want to multiply the fontsize with another value.

If you are fixing this, it would be great, if you also fix backgroundPosition and clip.

Landed fix for this:

http://github.com/jquery/jquery/commit/449e099b97d823ed0252d8821880bc0e471701ea