效果预览按下右侧的“点击预览”按钮可以在当前页面预览，点击链接可以全屏预览。https://codepen.io/comehope/pen/qYepNv可交互视频教程此视频是可以交互的，你可以随时暂停视频，编辑视频中的代码。请用 chrome, safari, edge 打开观看。https://scrimba.com/p/pEgDAM/cQ73Vt8源代码下载每日前端实战系列的全部源代码请从 github 下载：https://github.com/comehope/front-end-daily-challenges代码解读定义 dom，容器中包含文本：<div class="warning">ERROR 404</div>居中显示：body { margin: 0; height: 100vh; display: flex; align-items: center; justify-content: center; background-color: rgb(20%, 20%, 20%);}定义文字样式：.warning { color: whitesmoke; font-size: 100px; font-family: sans-serif; font-weight: bold;}用伪元素定义边框尺寸：.warning { position: relative; padding: 0.6em 0.4em;}.warning::before,.warning::after { content: ''; position: absolute; top: 0; left: 0; width: 100%; height: 100%; border: 0.2em solid; box-sizing: border-box;}把边框分为两部分，拼在一起：.warning::before,.warning::after { border: 0.2em solid transparent; color: orangered;}.warning::before { border-top-color: currentColor; border-right-color: currentColor;}.warning::after { border-bottom-color: currentColor; border-left-color: currentColor;}把上边框和右边框下沉一层：.warning::before { z-index: -1;}为下边框和在边框加上阴影：.warning::after { box-shadow: 0.3em 0.3em 0.3em rgba(20%, 20%, 20%, 0.8);}最后，让边框转起来：.warning::before,.warning::after { animation: rotating 10s infinite;}@keyframes rotating { to { transform: rotate(360deg); }}大功告成！
