* {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
}

html, body {
    height: 100%;
    width: 100%;
}

#container {
    position: relative;
    height: 100%;
    width: 100%;
}

#moveBtn {
    position: absolute;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    padding: 10px 20px;
    font-size: 16px;
    cursor: pointer;
}

#rectangle {
    position: absolute;
    width: 100px;
    height: 80px;
    background-color: red;
}