两个问题，动画颤抖和元素定位

来源：2-18 编程练习

慕工程8451887

2019-08-18 06:56:25

问题一：动画运行时会有一个左右颤抖的效果，如何能避免呢？

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>2-5</title>
    <style type="text/css">
        div {
            position: absolute;
            top: 0;
            right: 0;
            bottom: 0;
            left: 0;
            box-sizing: border-box;
            width: 400px;
            height: 400px;
            margin: auto;
            border: 1px solid red;
            border-bottom: 200px solid red;
            border-radius: 50%;
            transform-origin: 50% 50%;
            animation-name: rotate;
            animation-duration: 5s;
            animation-timing-function: linear;
            /*此处写代码*/
            animation-iteration-count: infinite;
        }
        
        div::before {
            position: absolute;
            content: "";
            top: 50%;
            margin: auto;
            box-sizing: border-box;
            width: 200px;
            height: 200px;
            background-color: red;
            border:80px solid white;
            border-radius: 50%;
        }
        
        div::after {
            position: absolute;
            content: "";
            top: 50%;
            left: 200px;
            margin: auto;
            box-sizing: border-box;
            width: 200px;
            height: 200px;
            background-color: white;
            border:80px solid red;
            border-radius: 50%;
        }
        
        @keyframes rotate {
            from {transform: rotate(0deg);}
            to   {transform: rotate(360deg);
    </style>
</head>
<body>
    <div></div>
</body>
</html>

问题二：在用 after 和 before 伪元素的时候，设置 top:0; bottom:0; margin: auto; 无法实现伪元素高度居中，而使用 top:50%可以实现高度居中。这是为什么呢？？下面是一个示例代码

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>2-5</title>
    <style type="text/css">
        div {
            position: absolute;
            top: 0;
            right: 0;
            bottom: 0;
            left: 0;
            box-sizing: border-box;
            width: 400px;
            height: 400px;
            margin: auto;
            border: 1px solid red;
            border-bottom: 200px solid red;
            border-radius: 50%;
        }
        
        div::after {
            position: absolute;
            content: "";
            top: 0;
            right: 0;
            bottom: 0;
            left: 0;
            box-sizing: border-box;
            width: 200px;
            height: 200px;
            margin: auto;
            border: 1px solid green;
            border-bottom: 200px solid green;
            border-radius: 50%;
        }
        
    </style>
</head>
<body>
    <div></div>
</body>
</html>

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