CSS: offsetTop offsetLeft offsetParent

offsetParent is containing block

1. position: static; offsetTop 元素的上外边距到containing block的上内边距 (containing block的padding + element.margin)

<!DOCTYPE html>
<html lang="en-US">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <section>
        <article></article>
    </section>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        section {
            width: 300px;
            height: 3%;
            margin: 10px;
            border: 20px dotted peru;
            padding: 30px;
            background-color: tan;
            position: relative;
        }

        article {
            width: 100px;
            height: 100px;
            margin: 10px;
            border: 20px dashed wheat;
            padding: 30px;
            background-color: aqua;
        }
    </style>
    <script>
        const section = document.querySelector('section')
        const article = document.querySelector('article')
    </script>
</body>

</html>

2. position: absolute; 先理解top(margin-top到containing block的内边距), 因此, offsetTop = top + margin-top

<!DOCTYPE html>
<html lang="en-US">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <section>
        <article></article>
    </section>
    <style>
        /* * {
            margin: 0;
            padding: 0;
        } */

        section {
            width: 300px;
            height: 400px;
            margin: 10px;
            border: 20px dotted peru;
            padding: 30px;
            background-color: tan;
            position: relative;
        }

        article {
            width: 100px;
            height: 100px;
            margin: 10px;
            border: 20px dashed wheat;
            padding: 30px;
            background-color: aqua;
            position: absolute;
            top: 10px;
            left: 20px;
        }
    </style>
    <script>
        const section = document.querySelector('section')
        const article = document.querySelector('article')
    </script>
</body>

</html>