index.html

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width,user-scalable=no,initial-scale=1">
  <title>Get Magnet</title>
  <style>
    @import url(https://fonts.googleapis.com/css?family=Fira+Sans:400,300italic,400italic,300);

    html,
    body {
      margin: 0;
      padding: 0;
      height: 100%;
      font: 16px 'Fira Sans';
      color: #444;
    }

    body {
      display: flex;
      align-items: center;
      justify-content: center;
    }

    .button {
      display: block;
      border: 0;
      padding: 13px 14px;
      border-radius: 4px;
      color: #fff;
      font: inherit;
      font-style: italic;
      font-weight: 400;
      font-size: 27px;
      text-align: center;
      line-height: 1;
      cursor: pointer;
      text-decoration: none;
      margin: 20px;

      background: #49A3FF;
      box-shadow:
        0px 1px 3px 0px rgba(0,0,0,0.21),
        0px 2px 10px 0px rgba(0,0,0,0.13);
      border-radius: 4px;

      -webkit-user-select: none;
         -moz-user-select: none;
              user-select: none;
    }

    .button:active {
      opacity: 0.7;
      box-shadow: none;
      transform: translateY(1px);
    }
  </style>
</head>
<body>
<script>
  (function() {

    var render = {
      android: function() {
        var a = document.createElement('a');
        a.href = 'https://github.com/mozilla-magnet/magnet-client/releases/download/nightly/app-release.apk';
        a.textContent = 'Download Magnet';
        a.className = 'button';
        return a;
      },

      ios: function() {
        window.location = 'https://docs.google.com/a/mozilla.com/forms/d/1SP_W5warjpvvJahLsEZ5-QdMLJGSiy0ceGQsqRf5JPM/viewform';
      },

      unsupported: function() {
        var el = document.createElement('h3');
        el.textContent = 'Your device is not supported';
        return el;
      }
    };

    function platform() {
      var ua = navigator.userAgent;
      if (/Android/.test(ua)) return 'android';
      else if (/iPhone/.test(ua)) return 'ios';
      else return 'unsupported';
    }


    var el = render[platform()]();
    if (el) document.body.appendChild(el);
  })();
</script>
</body>
</html>