Detect if Windows font rendering is via DirectWrite or GDI

HTML

<div id="outercontainer">
    <div id="innercontainer">Lorem ipsum dolor sit amet</div>
</div>

CSS

#outercontainer {
    font-size: 50px;
}
#innercontainer {
    float: left;
    height: 9px;
    overflow: auto;
    font-size: 33.3333%;
    line-height: 3;
}

JavaScript

$(document).ready(function(){

    var elem = document.getElementById("innercontainer");
    var width = window.getComputedStyle ? window.getComputedStyle(elem,null).getPropertyValue("width") : '36px';  
    if(width == '36px'){
        alert('Font-rendering uses Windows GDI');
    }
    else {
        alert('Font-rendering uses either DirectWrite on Windows or an equivalent technique on Mac/Linux');
    }

});