// 使用jQuery内置的fade方法
$('#element').fadeIn('slow');
$('#element').fadeOut('slow');

$('#element').animate({
    opacity: 0.5
}, {
    duration: 500,
    step: function(now, fx) {
        if ($.browser.msie && parseInt($.browser.version, 10) <= 8) {
            this.style.filter = 'alpha(opacity=' + now * 100 + ')';
        } else {
            this.style.opacity = now;
        }
    }
});

if (!Modernizr.opacity) {
    // IE8及以下浏览器的处理
    $.fn.animateOpacity = function(to, duration, callback) {
        return this.each(function() {
            var elem = $(this);
            elem.stop().animate({ opacity: to }, {
                duration: duration,
                step: function(now) {
                    elem[0].style.filter = 'alpha(opacity=' + (now * 100) + ')';
                },
                complete: callback || function() {}
            });
        });
    };
}