lovecoding
/
W250913


			
				
					
						
						
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							<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box1{
            width: 400px;
            height: 400px;
            border:3px dashed red;
            position: relative;
        }
        /* 父元素 子元素 大小都确定的情况下 */
        .box2{
            width: 200px;
            height: 200px;
            background-color: blue;
            position: relative;
            top:100px;
            left: 100px;
        }
        /* 父元素未知大小 子元素大小已知 */
        .box3{
            width: 200px;
            height: 200px;
            background-color: green;
            position: absolute;
            top:50%;
            left: 50%;
            margin-top: -100px;
            margin-left: -100px;
        }
    </style>
</head>
<body>
    <div class="box1">
        <div class="box2"></div>
    </div>
    <div class="box1">
        <div class="box3"></div>
    </div>
</body>
</html>