function qs(el, sel) {
  var elements;

  if (!el) {
    throw new Error("No Element Provided");
  }

  if (typeof el.querySelector != "undefined") {
    return el.querySelector(sel);
  } else {
    elements = el.getElementsByTagName(sel);

    if (elements.length) {
      return elements[0];
    }
  }
}
querySelector polyfill
@param {element} el
@param {string} sel selector string
@returns {element} element
@memberof Core