html,
body {
    background-color: #F2F4F7;
}

.banner {
    width: 100%;
    height: 600px;
    overflow: hidden;
    position: relative;
}
.banner img {
    width: 1920px;
    height: 600px;
    position: absolute;
    left: 50%;
    top: 0;
    transform: translateX(-50%);
}

.case-list li {
    float: left;
    width: 392px;
    height: 220px;
    position: relative;
    overflow: hidden;
    margin: 0 12px 10px 0;
}
.case-list li:nth-child(3n) {
    margin-right: 0;
}
.case-list li a {
    display: block;
    position: relative;
}
.case-list li a img {
    display: block;
    width: 100%;
}
.case-list li a:hover .cover {
    position: absolute;
    left: 0;
    top: 0;
    width: 100%;
    height: 100%;
    background-color: rgba(0, 0, 0, 0.05);
}