63.1拓展之纯 CSS 创作一个摇摇晃晃的 loader

效果地址：https://scrimba.com/c/cqKv4VCR

 HTML code:

<div class="loader">
    <span>Loading...</span>
</div>

CSS code:

html, body {
    margin: 0;
    padding: 0;
}
/* 设置body的子元素水平垂直居中 */
body {
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: black;
    overflow: hidden;
}
/* 设置容器尺寸 */
.loader {
    /* width、height包括边框、内边距、内容区 */
    box-sizing: border-box;
    position: relative;
    /* 在此调节font-size可以调节整个loader的大小，前提是不能在后代元素里设置font-size了 */
    font-size: 20px;
    width: 10em;
    height: 10em;
    border-radius: 50%;
    border-top: 0.3em solid red;
    border-bottom: 0.3em solid blue;
    animation: rotating 2s ease-in-out infinite;
    --direction: 1;
}
/* 设置文字样式 */
.loader span {
    position: absolute;
    color: white;
    width: inherit;
    height: inherit;
    text-align: center;
    line-height: 10em;
    font-family: sans-serif;
    animation: rotating 2s linear infinite;
    --direction: -1;
}
@keyframes rotating {
    50% {
        transform: rotate(calc(180deg * var(--direction)));
    }

    100% {
        transform: rotate(calc(360deg * var(--direction)));
    }
}