function isActiveElement(el, { deep = false } = {}) {
  const activeEl = deep ? getDeepActiveElement() : document.activeElement;
  return el === activeEl;
}
Readable alternative for `expect(el).to.equal(document.activeElement);`.
While this is readable by itself, it makes Web Test Runner hang completely in many occasions.
Therefore it's better to write:
`expect(isActiveElement(el)).to.be.true;`
@param {Element} el
@param {{deep?: boolean}} opts
@returns {boolean}