HTML CSS Center DIV

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>My Centered Tag</title>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <style>
      html *{
        font-size: 35px !important;
        margin: 2px;
      }
      html,
      body {
        height: 100%;
      }

      @media only screen and (min-width: 600px) {
        body {
          margin-right:200px;
          margin-left:200px;
          margin-top: 0;
        }
      }
      .flex{
        display: grid;
        grid-row-gap: 1rem;
        grid-template-columns: repeat(auto-fit, minmax(150px, 1fr));
      }

      .grid{
        display: grid;
        place-items: center;
      }

    </style>
    <script>
      document.addEventListener("DOMContentLoaded", function(){

      });
    </script>
  </head>
  <body>
    <div class="grid" style="height:100%">
      <div>
        <button class="button">Button #1</button>
        <button class="button">Button #2</button>
      </div>
    </div>
    <div class="grid" style="height:100%">
      <div>
        <button class="button">Button #1</button>
        <button class="button">Button #2</button>
      </div>
    </div>
  </body>
</html>