style.css

body {
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background: linear-gradient(dimgray, black);
}

.court {
    width: 20em;
    height: 20em;
    border: 1em solid silver;
    background-color: black;
    border-radius: 1em;
    position: relative;
}

.paddle {
    width: 1em;
    height: 9em;
    background-color: white;
    border-radius: 0.5em;
    position: absolute;
    top: 1em;
}

.paddle.left {left: 1em;}
.paddle.right {right: 1em;}

.ball {
    width: 1em;
    height: 1em;
    background-color: red;
    border-radius: 50%;
    position: absolute;
    left: 2em;
    top: 8.5em;
}

.paddle,
.ball {
    animation: 1s linear infinite alternate;
}

.paddle {animation-name: moving;}
.ball {animation-name: bounce;}
.paddle.right {animation-delay: -1s;}

@keyframes moving {
    to {
        transform: translateY(9em);
    }
}

@keyframes bounce {
    to {
        transform: translateX(15em);
    }
}