index.html

<!DOCTYPE html>
<html>
<head>
  <style>
    #shortcutkeys {
      width: 35%;
      height: 80%;
      display: none;
      background-color: whitesmoke;
      box-shadow: 0 7px 15px 3px black;
      border-radius: 5px;
      padding-top: 1em;
      position: fixed;
      top: 10%;
      left: 30%; z-index: 9;
      overflow-y: auto; overflow-x: hidden;
    }
    .boardkey {
      margin: 5px;
      text-align: center; align-self: center;
      width: 25px;
      height: 25px;
      background: white;
      box-shadow: 0 0 3px grey;
      border-bottom: 3px solid grey;
      color: black;
      border-radius: 5px;
    }
    .shortcutlist {
      width: 100%;

    }
    .nothin {
      width: 100%; 
    }
    .innershortcut {
      display: flex; justify-content: flex-end;
      text-align: right;
    }
#closeshortcut { user-select: none;
width: 40px; height: 40px; border: 1px solid grey; transform: rotate(45deg); font-weight: bold; font-size: 1.8em; border-radius: 50%; position: absolute; color: red; top: 2px; right: 2px;
}
#closeshortcut:hover {background: red; color: white; transition: 0.6s;}

#shortcutbg { top: 0px; left: 0px;
width: 100%; display: none; height: 100%; background: rgba(0,0,0,0.5); z-index: 8; position:fixed;
}
  </style>
  <title>Keyboard Shortcuts</title>
</head>
<body>

<div id="shortcutbg"></div>
<div id="shortcutkeys">
<button id="closeshortcut">+</button>

  <center><h2>Keyboard Shortcuts</h2></center>
  <table class="shortcutlist">
    <tr class="nothin">
      <td class="innershortcut">
        <div class="boardkey">f</div>
      </td>
      <td>Flip board</td>
    </tr>
    <tr class="nothin">
      <td class="innershortcut">
        <div class="boardkey">←</div>
        <div style="font-size: 1.5em">/</div>
        <div class="boardkey">→</div>
      </td>
      <td style="width: 50%;">Move backward/forward</td>
    </tr>
    <tr class="nothin">
      <td class="innershortcut">
        <div class="boardkey">↑</div>
        <div style="font-size: 1.5em">/</div>
        <div class="boardkey">↓</div>
      </td>
      <td style="width: 50%;">Go to start/end</td>
    </tr>
    <tr class="nothin">
      <td class="innershortcut">
        <div class="boardkey">?</div>
      </td>
      <td style="width: 50%;">Show this help dialog</td>
    </tr>
  </table>
<div style="width: 100%; height: 40px; color: white; background: goldenrod; font-weight: bolder; text-align: center;line-height: 40px; margin-top: 10px;">Mouse tricks</div>
<ul>
<li>You can also scroll over the board to move in the game.
<li>Press shift+click or right-click to draw circles and arrows on the board
<ul>

</div>

<script>
var closeThatKeyBox = document.getElementById('closeshortcut');
var bgdialog = document.getElementById('shortcutbg');

  closeThatKeyBox.addEventListener('click', function() {
document.getElementById('shortcutkeys').style.display = 'none';  
bgdialog.style.display = 'none';
  });

  bgdialog.addEventListener('click', function() {
document.getElementById('shortcutkeys').style.display = 'none';
bgdialog.style.display = 'none';
  });


  document.addEventListener("keydown", event => {
    if (event.key === "?") {
      document.getElementById('shortcutkeys').style.display = "block";
      document.getElementById('shortcutbg').style.display = "block";
    }
  });
</script>

</body>
</html>