Try to execute a Gist pre content and it copies it to clipboard.

prevaluation.js

document.documentElement.appendChild(
  document.createElement('style')
).textContent = `
.executable {
  transition: all .3s;
}
.executable:hover {
  cursor: pointer;
  background: #fff !important;
}`;
document.querySelectorAll('pre').forEach(pre => {
  try {
    const text = pre.textContent + '\n';
    const fn = Function(text);
    pre.onclick = () => {
      try { fn.call(window); } catch(meh) {}
      const ta = document.createElement('textarea');
      pre.parentNode.insertBefore(ta, pre);
      ta.value = text;
      ta.focus();
      ta.setSelectionRange(0, text.length);
      document.execCommand('copy');
      pre.parentNode.removeChild(ta);
    };
    pre.classList.add('executable');
  } finally {}
});