<!DOCTYPE html >
<html>
  <head>
    <meta charset="UTF-8"/>
    <style media="screen">
      body {
        background-color: #8A4358;
        color: #222;
        margin: 0;
        padding: 18px 0;
        text-align: center;
      }
      main {
        position: relative;
        margin: 0 auto;
        max-width: 870px;
        width: 100vw;
      }
      canvas {
        background-color: #4CC;
        cursor: none;
        width: 100%;
      }
    </style>

    <title>Tetrelementis</title>
  </head>
  <body>
    <main>
      <canvas id="canvas" width="900" height="630"></canvas>
    </main>
  </body>
  <script type="text/javascript" src="chemical-elements.js"></script>
  <script type="text/javascript" src="constants.js"></script>
  <script type="text/javascript" src="periodic-table.js"></script>
  <script type="text/javascript" src="tetromino.js"></script>
  <script type="text/javascript" src="preview-board.js"></script>
  <script type="text/javascript" src="tetris-board.js"></script>
  <script type="text/javascript" src="input.js"></script>
  <script type="text/javascript" src="render.js"></script>
  <script type="text/javascript" src="game.js"></script>
</html>

<!DOCTYPE html >
<html>
  <head>
    <meta charset="UTF-8"/>
    <style media="screen">
      body {
        background-color: #8A4358;
        color: #222;
        margin: 0;
        padding: 18px 0;
        text-align: center;
      }
      main {
        position: relative;
        margin: 0 auto;
        max-width: 900px;
        width: 100vw;
      }
      canvas {
        background-color: #4CC;
        cursor: none;
        width: 100%;
      }
    </style>

    <title>Tetrelementis</title>
  </head>
  <body>
    <main>
      <canvas id="canvas" width="900" height="630"></canvas>
    </main>
  </body>
  <script type="text/javascript" src="chemical-elements.js"></script>
  <script type="text/javascript" src="constants.js"></script>
  <script type="text/javascript" src="periodic-table.js"></script>
  <script type="text/javascript" src="tetromino.js"></script>
  <script type="text/javascript" src="preview-board.js"></script>
  <script type="text/javascript" src="tetris-board.js"></script>
  <script type="text/javascript" src="input.js"></script>
  <script type="text/javascript" src="render.js"></script>
  <script type="text/javascript" src="game.js"></script>
</html>

Increase main tag max width to full canvas width
Increase main tag max width to full canvas width
