getElementsByTagName.js

function getElementsByTagName(root, tagName) {
  tagName = tagName.toUpperCase();
  var stack = [root],
      result = [],
      current, node;
  while (stack.length) {
    current = stack.pop();
    for (var i = 0; i < current.children.length; i += 1) {
      node = current.children[i];
      if (node.tagName.toUpperCase() === tagName) {
        result.push(node);
      }
      stack.push(node);
    }
  }
  return result;
}