<!DOCTYPE html>

<html>
<head>
    <style>
        body {
            margin: 0;
        }
        .box {
            background-color: green;
            margin-top: 10px;
        }
        
        .width {
            width: 50vw;
            height: 100px;
        }
    </style>
</head>
<body>
<p>Both boxes should be the same width.</p>
<div class="box width"></div>
<div class="box width" style="height: 200px; margin-top: 20px"></div>
</body>
</html>