Untitled

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

    <link rel = "stylesheet" href = "styles.css">
</head>
<body>

    <style>
        #parent {
    width: 300px;
    border:1px solid red;
}

#child {
    height: 100px;
    border:1px solid green;
}

    </style>

    <div id="parent">
        <div id="child"></div>
    </div>

    <div class="a">

    </div>


</body>
</html>