css 背景图自适应

<style type="text/css">
    #weixinMain{
        width: 320px;
        height: 200px;
        background-image: url('../res/login/loginFrame.png');
        background-repeat:no-repeat;
        background-size:100% 100%;
        -moz-background-size:100% 100%;
    }
</style>