I went to the Internet for a long time and couldn't find it. Later, I thought of the cacl calculation attribute and solved it.
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
</head>
<style>
html,body{
height:100%;
}
.ani-box {
width: 100%;
height: 100%;
position: relative;
}
.ani {
width: 100px;
height: 100px;
background: #000;
animation: a 2s linear;
position: absolute;
top: 0;
left: 0;
}
@keyframes a {
0% {
top:0;
}
100% {
top:calc(100% - 100px);
}
}
</style>
<body>
<div class="ani-box">
<div class="ani"></div>
</div>
</body>
</html>finished~