grade-answer/css/index.css

* {
    padding: 0;
    margin: 0;
}
html,
body {
    width: 100%;
    height: 100%;
    box-sizing: border-box;
    overflow: hidden;
}
.container {
    width: 100%;
    height: 100%;
    background: url('../image/index/indexbg.jpg');
    background-size: 100% 100%;
    position: relative;
}
.btn {
    position: absolute;
    top: 60%;
    left: 50%;
    transform: translateX(-50%);
    width: 6.02rem;
    height: 1.98rem;
    background: url('../image/index/btn.png');
    background-size: 100% 100%;
}