placeholder兼容

<!------------placeholder兼容------------->
<script type="text/javascript">
    $(function () {
        if (!placeholderSupport()) {   // 判断浏览器是否支持 placeholder
            $('[placeholder]').focus(function () {
                var input = $(this);
                if (input.val() == input.attr('placeholder')) {
                    input.val('');
                    input.removeClass('placeholder');
                }
            }).blur(function () {
                var input = $(this);
                if (input.val() == '' || input.val() == input.attr('placeholder')) {
                    input.addClass('placeholder');
                    input.val(input.attr('placeholder'));
                }
            }).blur();
        };
    })
    function placeholderSupport() {
        return 'placeholder' in document.createElement('input');
    }

</script>
    <!------end------->