isElementInView = el => {
    const rect = el.getBoundingClientRect();
    return rect.top >= 0 && rect.left >= 0 && rect.bottom <= (window.innerHeight || document.documentElement.clientHeight) && rect.right <= (window.innerWidth || document.documentElement.clientWidth);
  }
Determines if the element is within the viewport.
@param el {HTMLElement}