Untitled

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
* { margin: 0; padding: 0}
.thumb_box {
  display: flex;
  flex-wrap: wrap;
}
.thumb {
  width: 45vw;
  height: 45vh;
  border: solid 1px #ccc;
  background:#e5e5e5;
  box-sizing:border-box;
}

</style>
</head>
<body>
    <div class="thumb_box">
        <div class="thumb">TEST</div>
        <div class="thumb">TEST</div>
      </div>
</body>
</html>