Testing for CSS 3D Transforms Support

has3d.js

function has3d(){
    if (!window.getComputedStyle) {
        return false;
    }
    
    var el = document.createElement('p'),
    has3d,
    transforms = {
        'webkitTransform':'-webkit-transform',
        'OTransform':'-o-transform',
        'msTransform':'-ms-transform',
        'MozTransform':'-moz-transform',
        'transform':'transform'
    };

    // Add it to the body to get the computed style
    document.body.insertBefore(el, null);

    for(var t in transforms){
        if( el.style[t] !== undefined ){
            el.style[t] = 'translate3d(1px,1px,1px)';
            has3d = window.getComputedStyle(el).getPropertyValue(transforms[t]);
        }
    }

    document.body.removeChild(el);

    return (has3d !== undefined && has3d.length > 0 && has3d !== "none");
}

For performance I'd go with something like:

var has3d = (function(){
    var el = document.createElement('p'),
    has3d,
    transforms = {
        'webkitTransform':'-webkit-transform',
        'OTransform':'-o-transform',
        'msTransform':'-ms-transform',
        'MozTransform':'-moz-transform',
        'transform':'transform'
    };

    // Add it to the body to get the computed style
    document.body.insertBefore(el, null);

    for(var t in transforms){
      if (transforms.hasOwnProperty(t)) {
        if( el.style[t] !== undefined ){
            el.style[t] = 'translate3d(1px,1px,1px)';
            has3d = window.getComputedStyle(el).getPropertyValue(transforms[t]);
          }
        }
    }

    document.body.removeChild(el);

    return (has3d !== undefined && has3d.length > 0 && has3d !== "none");
}());

Just so the above function only runs the once per page instead of whenever you need to detect if we have 3d...