父元素没有设置定位 position absolute 解析

1、示例代码

<!DOCTYPE html>
<html lang="zh">

    <head>
        <meta charset="UTF-8" />
        <meta name="viewport" content="width=device-width, initial-scale=1.0" />
        <meta http-equiv="X-UA-Compatible" content="ie=edge" />
        <title>父元素没有设置定位 position absolute 解析</title>
        <style type="text/css">
            html {
                border: 1px dashed green;
            }

            body {
                border: 1px dashed purple;
            }

            #first {
                width: 200px;
                height: 100px;
                border: 1px solid red;
                position: relative;
            }

            #second {
                width: 200px;
                height: 100px;
                border: 1px solid blue;
                position: absolute;
                top: 0;
                left: 0;
            }
        </style>
    </head>

    <body>
        <div id="first">relative</div>
        <div id="second">absoult</div>
    </body>

</html>

效果：

2、结论

absoulte方式在父级没有设置定位方式时是以<html>作为原点定位。