md.html

<!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>
        .second-child {
            width: 850px;
            /* change the width to 200px */
            height: 100%;
            /* change the height to 100px */
            /* add other styles as needed */
            background-color: black;
        }
    </style>
</head>

<body>
    <div class="parent">
        <div class="child">First child</div>
        <div class="child second-child">
        <?php
        if (isset($_GET['home'])) {
            include('home.php');
        } elseif (isset($_GET['bookmark'])) {
            include('message.php');
        } elseif (isset($_GET['message'])) {
            include('bookmark.php');
        } elseif (isset($_GET['more'])) {
            include('more.php');
        } elseif (isset($_GET['explore'])) {
            include('explore.php');
        } elseif (isset($_GET['profile'])) {
            include('profile.php');
        } elseif (isset($_GET['not'])) {
            include('not.php');
        } elseif (isset($_GET['list'])) {
            include('list.php');
        } else {
            include('home.php');
        }
        ?>
        </div>
        <!-- other child elements -->
    </div>
</body>

</html>