index.html

<div id="errors" style="
  background: #c00;
  color: #fff;
  display: none;
  margin: -20px -20px 20px;
  padding: 20px;
  white-space: pre-wrap;
"></div>
<div id="root"></div>
<script>
  window.addEventListener('mousedown', function(e) {
    document.body.classList.add('mouse-navigation');
    document.body.classList.remove('kbd-navigation');
  });
  window.addEventListener('keydown', function(e) {
    if (e.keyCode === 9) {
      document.body.classList.add('kbd-navigation');
      document.body.classList.remove('mouse-navigation');
    }
  });
  window.addEventListener('click', function(e) {
    if (e.target.tagName === 'A' && e.target.getAttribute('href') === '#') {
      e.preventDefault();
    }
  });
  window.onerror = function(message, source, line, col, error) {
    var text = error ? error.stack || error : message + ' (at ' + source + ':' + line + ':' + col + ')';
    errors.textContent += text + '\n';
    errors.style.display = '';
  };
  console.error = (function(old) {
    return function error() {
      errors.textContent += Array.prototype.slice.call(arguments).join(' ') + '\n';
      errors.style.display = '';
      old.apply(this, arguments);
    }
  })(console.error);
</script>