HTMLElement.accessKeyLabel

label = element.accessKeyLabel

var btn = document.getElementById('btn1');
var shortcutLabel = btn.accessKeyLabel || btn.accessKey;
btn.title += ' [' + shortcutLabel.toUpperCase() + ']';

btn.onclick = function () {
  var feedback = document.createElement('output');
  feedback.textContent = 'Pressed!';
  btn.insertAdjacentElement('afterend', feedback);
};

<button accesskey="h" title="Caption" id="btn1">Hover me</button>