Adds a copy button after the tags you define (like code or pre). Clicking the button copies the preceeding element/tag text content to the OS clipboard.

JavaScript: Copy Button for Defined Tags

function reactionaryCopybutton() {
  var container = arguments.length <= 0 || arguments[0] === undefined ? document.getElementsByTagName('body')[0] : arguments[0];
  var tags = arguments.length <= 1 || arguments[1] === undefined ? ['blockquote', 'pre', 'code'] : arguments[1];

  var errorExplanation = 'Try to invoke reactionaryCopybutton() without parameters, or write something like this:';
  var codesuggestion = '\tvar main = document.getElementById(\'#yourContentContainer\');\n\tvar tags = [\'pre\'];\n\treactionaryCopybutton(main, tags);';
  try {
    container.nodeType !== 1;
    container['typeof'] !== 'undefined';
    container !== 'NULL';
  } catch (err) {
    console.error(err + ' \n%cFirst argument must be a DOM node (or undefined).', 'font-weight: bold');
    console.info(errorExplanation + '\n %c' + codesuggestion, 'color: #ababab');
  };

  try {
    if (!Array.isArray(tags)) {
      throw 'Tags is not an array';
    };
  } catch (err) {
    console.error(err + ' \n%cSecond argument must be an Array of HTML tags (or empty)', 'font-weight: bold');
    console.info(errorExplanation + '\n %c' + codesuggestion, 'color: #ababab');
  };

  if (document.execCommand) {
    var hiddenCopyContainer = document.createElement('TEXTAREA');
    hiddenCopyContainer.style.position = 'absolute';
    hiddenCopyContainer.style.left = '-200vw';
    hiddenCopyContainer.id = 'hiddenCopyContainer';
    window.body.appendChild(hiddenCopyContainer);

    var blocks = container.querySelectorAll(tags.join());
    if (blocks.length > 0) {
      for (var i = 0; i < blocks.length; i++) {
        var copyButton = document.createElement('BUTTON');
        copyButton.className = 'reactionary-copy-button';
        copyButton.innerHTML = '<h5>Copy</h5>';
        copyButton.addEventListener('click', reactionaryCopyClickListener.bind(null, blocks[i].innerText));
        blocks[i].appendChild(copyButton);
      }
    }
  }
}

function reactionaryCopyClickListener(textToCopy) {
  var hiddenCopyContainer = document.getElementById('hiddenCopyContainer');
  hiddenCopyContainer.value = textToCopy;
  hiddenCopyContainer.select();
  document.execCommand('copy');
}

Works in everything except IE<9, which is pretty good, and Safari.
If it does not work, no copy button(s) are displayed, so no loss.

IE and Edge has a "skipping bug"; it scrolls to the bottom of the page when a copy button is clicked. This is related to the position of hiddenCopyContainer (IE scrolls to the position of the selected input element, and in this case it's positioned at the bottom of the page (and off to the left a good ways).

Buttons are unstyled, but can be access bu the CSS class .reactionary-copy-button

Updated:
The first version was in EcmaScript 2015, so I figured it was a good idea to transpile it before sharing :)
Also added arguments, so rather than changing settings inside the function, it now accepts arguments.
And now that it accepts arguments, it needed to vet those arguments, so there are now tests for each of the arguments!

The two arguments are:
1: container - A DOM element. Limits the part of the document that the function affects. Default is the body node.
2: tags - An array of tag names that determines what element types (inside "container") that will receive copy buttons. Default is ['pre','code','blockquote'].