17.纯 CSS 创作炫酷的同心矩形旋转动画

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

html, body ,.loader{
    margin: 0;
    padding: 0;
    height: 100%;
    display: flex;
    justify-content: center;
    align-items: center;
    background-color: black;
}
/* 设置span的样式 */
.loader{
    position: relative;
    width: 10em;
    height: 10em;
    font-size: 28px;
}
.loader span{
    position: absolute;
    width: 100%;
    height: 100%;
    /* 0.3表示透明度 */
    background-color: rgba(100%, 0%, 0%, 0.3);
    /* 并排 */
    box-sizing: border-box;
    border: 0.5em solid;
    /* border-color: 上下  左右 */
    border-color: white rgba(100%, 100%, 100%, 0.2);
    /* 动画名称 周期 节奏 循环次数 是否反向播放 */
    animation: animate 5s ease-in-out infinite alternate;
}
@keyframes animate{
    0%{
        /* red */
        /* 颜色变换 */
        background-color: rgba(100%, 0%, 0%, 0.3);
        /* 旋转 */
        transform: rotate(0deg);
    }
    25% {
        /* yellow */
        background-color: rgba(100%, 100%, 0%, 0.3);
    }
    50% {
        /* green */
        background-color: rgba(0%, 100%, 0%, 0.3);
    }
    75% {
        /* blue */
        background-color: rgba(0%, 0%, 100%, 0.3);
    }
    100% {
        /* purple */
        background-color: rgba(100%, 0%, 100%, 0.3);
        transform: rotate(720deg);
    }
}
/* 分别设置5个span的尺寸 */
.loader span:nth-child(1){
    width: calc(20% + 20% * (5-1));
    height: calc(20% + 20% * (5-1));
    /* 为每个span设置动画延时 */
    animation-delay: calc(0.2s * (5-1));
}
.loader span:nth-child(2) {
    width: calc(20% + 20% * (5 - 2));
    height: calc(20% + 20% * (5 - 2));
    animation-delay: calc(0.2s * (5 - 2));
}
.loader span:nth-child(3) {
    width: calc(20% + 20% * (5 - 3));
    height: calc(20% + 20% * (5 - 3));
    animation-delay: calc(0.2s * (5 - 3));
}
.loader span:nth-child(4) {
    width: calc(20% + 20% * (5 - 4));
    height: calc(20% + 20% * (5 - 4));
    animation-delay: calc(0.2s * (5 - 4));
}
.loader span:nth-child(5) {
    width: calc(20% + 20% * (5 - 5));
    height: calc(20% + 20% * (5 - 5));
    animation-delay: calc(0.2s * (5 - 5));
}