body {
    padding: 100px;
    background: #717171;
}

#slider {
    width: 400px;
    overflow: hidden;
    border: 10px solid #4C4C4C;
    height: 266px;
    position: relative;
    margin: auto;
}

#slider ul {
    list-style-type: none;
    margin: 0;
    padding: 0;
    position: absolute;
    top: 0;
    left: 0;
    display: flex;
}

#slider ul li img {
    display: block;
}