Is it possible to detect animated gif images client side?

I just wrote some JS that detects animated gifs. Works in most modern browsers except IE 9.

Disclaimer: this only works if the domain origin of the image is the same as the page you are loading the script from.

See the gist for the latest version of the code: https://gist.github.com/3012623

function isAnimatedGif(src, cb) {
    var request = new XMLHttpRequest();
    request.open('GET', src, true);
    request.responseType="arraybuffer";
    request.addEventListener('load', function () {
        var arr = new Uint8Array(request.response),
            i, len, length = arr.length, frames = 0;

        // make sure it's a gif (GIF8)
        if (arr[0] !== 0x47 || arr[1] !== 0x49 || 
            arr[2] !== 0x46 || arr[3] !== 0x38)
        {
            cb(false);
            return;
        }

        //ported from php http://www.php.net/manual/en/function.imagecreatefromgif.php#104473
        //an animated gif contains multiple "frames", with each frame having a 
        //header made up of:
        // * a static 4-byte sequence (\x00\x21\xF9\x04)
        // * 4 variable bytes
        // * a static 2-byte sequence (\x00\x2C) (some variants may use \x00\x21 ?)
        // We read through the file til we reach the end of the file, or we've found 
        // at least 2 frame headers
        for (i=0, len = length - 9; i < len, frames < 2; ++i) {
            if (arr[i] === 0x00 && arr[i+1] === 0x21 &&
                arr[i+2] === 0xF9 && arr[i+3] === 0x04 &&
                arr[i+8] === 0x00 && 
                (arr[i+9] === 0x2C || arr[i+9] === 0x21))
            {
                frames++;
            }
        }

        // if frame count > 1, it's animated
        cb(frames > 1);
    });
    request.send();
}