style.css

body {
    margin: 0;
    overflow: hidden;
    background-color: black;
}

#game-area {
    position: relative;
    width: 100vw;
    height: 100vh;
}

#player {
    position: absolute;
    bottom: 20px;
    left: 50%;
    transform: translateX(-50%);
    width: 50px;
    height: 30px;
    background-color: green;
}

.invader {
    position: absolute;
    width: 40px;
    height: 40px;
    background-color: red;
}

.bullet {
    position: absolute;
    width: 5px;
    height: 15px;
    background-color: yellow;
}