低版本浏览器支持placeholder属性

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');
    }
    if (input[type = 'password'].val() == input[type = 'password'].attr('placeholder')) {
        input[type = 'password'].val('');
        input[type = 'password'].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');
    }