style.css

body {
    display: flex;
}

.game-container {
    width: 500px;
    height: 730px;
    position: absolute;
    left: 80px;
}

.border-left {
    width: 140px;
    height: 820px;
    position: absolute;
    background-color: white;
    z-index: +4;
}

.border-right {
    width: 140px;
    height: 820px;
    position: absolute;
    background-color: white;
    z-index: +4;
    left: 580px;
}

.border-top {
    width: 660px;
    height: 80px;
    position: absolute;
    background-color: white;
    z-index: +4;
    top: -50px;
}

.sky {
    background-color: skyblue;
    width: 500px;
    height: 580px;
    position: absolute;
}

.ground {
    background-image: url('assets/grass_ground.png');
    background-size: cover;
    width: 500px;
    height: 150px;
    position: absolute;
    top: 580px;
    z-index: +3;
}

.duck {
    background-image: url('assets/duck.png');
    background-size: cover;
    position: absolute;
    width: 60px;
    height: 45px;
}

.obstacle {
    background-image: url('assets/tree_stump.png');
    background-size: contain;
    width: 60px;
    height: 300px;
    position: absolute;
    z-index: +2;
}

.topObstacle {
    background-image: url('assets/tree_stump.png');
    background-size: contain;
    transform: rotate(180deg);
    width: 60px;
    height: 300px;
    position: absolute;
    z-index: +2;
}

.cloud {
    background-image: url('assets/cloud.png');
    background-size: cover;
    width: 140px;
    height: 110px;
    position: absolute;
    z-index: +1;
}

h1 {
    color: red;
    z-index: +5;
    position: absolute;
    font-size: 124px;
}