index.html

<!DOCTYPE html>
<html>
  <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>RN Web</title>
    <style>
      #app-root {
        display: flex;
        flex: 1 1 100%;
        height: 100vh;
      }
    </style>
  </head>
  <body>
    <div id="app-root"></div>
  </body>
</html>