点击标签，焦点转移到关联的表单字段

function focusLabels() {
    if (!document.getElementsByTagName) return false;
    var labels = document.getElementsByTagName("label");
    for (var i=0;i<labels.length;i++) {
        if (!labels[i].getAttribute("for")) continue;
        labels[i].onclick = function () {
            var id = this.getAttribute("for");
            if (!document.getElementById(id)) return false;
            var element = document.getElementById(id);
            element.focus();
        }
    }
}