如何div中的div居中 css

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Flexbox Centering</title>
    <style>
        .outer-div {
            display: flex;
            justify-content: center; /* 水平居中 */
            align-items: center; /* 垂直居中 */
            height: 300px; /* 设置高度以便观察效果 */
            border: 1px solid black; /* 添加边框以便观察效果 */
        }
        .inner-div {
            width: 100px;
            height: 100px;
            background-color: red; /* 设置背景颜色以便观察效果 */
        }
    </style>
</head>
<body>
    <div class="outer-div">
        <div class="inner-div"></div>
    </div>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Grid Centering</title>
    <style>
        .outer-div {
            display: grid;
            place-items: center; /* 水平和垂直居中 */
            height: 300px; /* 设置高度以便观察效果 */
            border: 1px solid black; /* 添加边框以便观察效果 */
        }
        .inner-div {
            width: 100px;
            height: 100px;
            background-color: blue; /* 设置背景颜色以便观察效果 */
        }
    </style>
</head>
<body>
    <div class="outer-div">
        <div class="inner-div"></div>
    </div>
</body>
</html>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Absolute Positioning Centering</title>
    <style>
        .outer-div {
            position: relative;
            height: 300px; /* 设置高度以便观察效果 */
            border: 1px solid black; /* 添加边框以便观察效果 */
        }
        .inner-div {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%); /* 水平和垂直居中 */
            width: 100px;
            height: 100px;
            background-color: green; /* 设置背景颜色以便观察效果 */
        }
    </style>
</head>
<body>
    <div class="outer-div">
        <div class="inner-div"></div>
    </div>
</body>
</html>