如何使用纯CSS实现在容器中反弹的小球（附源码）

<div class="loader"></div>

body {
    margin: 0;
    height: 100vh;
    display: flex;
    align-items: center;
    justify-content: center;
    background-color: black;
}

.loader {
    width: 10em;
    height: 3em;
    border: 0.3em solid silver;
    border-radius: 3em;
    font-size: 20px;
}

.loader {
    border-left-color: hotpink;
    border-right-color: dodgerblue;
}

.loader {
    position: relative;
}

.loader::before {
    content: '';
    position: absolute;
    top: 0;
    left: 0;
    width: 3em;
    height: 3em;
    border-radius: 50%;
    background-color: dodgerblue;
}

.loader::before {
    animation: shift 3s linear infinite;
}

@keyframes shift {
    50% {
        left: 7em;
    }
}

.loader::before {
    animation:
        shift 3s linear infinite,
        change-color 3s linear infinite;
}

@keyframes change-color {
    0%, 55% {
        background-color: dodgerblue;
    }

    5%, 50% {
        background-color: hotpink;
    }
}

.loader {
    animation: spin 3s linear infinite;
}

@keyframes spin {
    to {
        transform: rotate(360deg);
    }
}