docs: function isElementInViewport(el, inCB, outCB, rootMargin) {
    var margin = rootMargin || '-10%';
    function handleIntersect(entries, observer) {
      var entry = entries[0];
      if (entry.isIntersecting) {
        if (inCB && typeof inCB === 'function') inCB(el, entry);
      } else {
        if (outCB && typeof outCB === 'function') outCB(el, entry);
      }
    }
    var observer = new IntersectionObserver(handleIntersect, {rootMargin: margin});
    observer.observe(el);
  }