图片下落效果

利用css的 animation 实现
代码如下
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<style>
    .san{
        left: 10%;
        top: 20%;
        -webkit-animation:dn400 3s 0s ease both;
        -moz-animation:dn400 3s 0s ease both;
 /*使用dn400 这个效果*/
        animation:dn400 3s 0s ease both;
    }
 /*定制效果*/
    @-webkit-keyframes dn400{0%{opacity:0;-webkit-transform:translateY(-400px);}
        100%{opacity:1;-webkit-transform:translateY(0);}}
    @-moz-keyframes dn400{0%{opacity:0;-moz-transform:translateY(-400px);}
        100%{opacity:1;-moz-transform:translateY(0);}}
    @-o-keyframes dn400{0%{opacity:0;-o-transform:translateY(-400px);}
        100%{opacity:1;-o-transform:translateY(0);}}
    @keyframes dn400{0%{opacity:0;transform:translateY(-400px);}
        100%{opacity:1;transform:translateY(0);}}
</style>
<body>
    <img src="static/images/san.png" alt="" class="san" />
    <form action="">

        <input type="submit" value="提交" >
    </form>


</body>
</html>