tut_29.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Size Units</title>
    <style>
        .container{
            /* width: 500px; */
            /* height: 700px; */
            height: 100vh;
            width: 100vw;
            font-size: 10px;
            border: 2px solid red;
        }
        h1{
            text-align: center;
            font-size: 2rem;
        }
        #first{
            font-size: 3rem;
            padding: 3rem;
        }
    </style>
</head>
<body>
   <div class="container">
       <h1 id="first">Thus is first heading</h1>
       <h1 id="second">This is  second heading</h1>
       <h1 id="third">This is third heading</h1>
   </div>  
</body>
</html>