I am setting a css class to some select options in JS. This class includes margin style. It is working in the FF but not in IE and chrome.

 window.onload = function() {
    replace('edit-field-region-tid');
    replace('edit-tid');
}
function replace(id) {
    var i = 0;
    var s = document.getElementById(id);
    for (i; i < s.options.length; i++) {
        if (find(s.options[i].text, id, i)) {
            s.options[i].setAttribute("class", "sub_options");
        }
    }
}
function find(str, id, option_id) {
    var i;
    var s = document.getElementById(id);
    for (i = 0; i < str.length; i++) {
        if (str.charAt(i) == '-') {
            s.options[option_id].text = str.cutAt(0, "");
            return true;
        }
    }
    return false;
}
String.prototype.cutAt = function(index, char) {
        return this.substr(index+1, this.length);
}

And CSS:

.sub_options{
    margin-left:20px;
    text-indent:-2px;
}

Any ideas thanks!