css页面整体居中

<!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>
        body, html {
            height: 100%;
            margin: 0;
        }
        .container {
            display: flex;
            justify-content: center;
            align-items: center;
            height: 100%;
        }
        .centered-content {
            text-align: center;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="centered-content">
            <h1>Hello, World!</h1>
        </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>
        body, html {
            height: 100%;
            margin: 0;
        }
        .container {
            display: grid;
            place-items: center;
            height: 100%;
        }
        .centered-content {
            text-align: center;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="centered-content">
            <h1>Hello, World!</h1>
        </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>Traditional Centering</title>
    <style>
        body, html {
            height: 100%;
            margin: 0;
        }
        .container {
            position: relative;
            height: 100%;
        }
        .centered-content {
            position: absolute;
            top: 50%;
            left: 50%;
            transform: translate(-50%, -50%);
            text-align: center;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="centered-content">
            <h1>Hello, World!</h1>
        </div>
    </div>
</body>
</html>