var myFrame = document.getElementById('myFrame'),

    frameBody = myFrame.contentDocument.body,

    btn = document.querySelector('button'),

    op = document.querySelector('output');

​

frameBody.innerHTML = 'Click and edit the contents of this frame. Use ctrl-b and ctrl-i to add bold or italic.';

​

myFrame.contentDocument.designMode = 'on';

​

btn.addEventListener('click', function () {

  op.innerHTML = frameBody.innerHTML;

}, false);