Михаил

document.addEventListener('DOMContentLoaded', () => {
  first();
  setTimeout(second, 10);
  setTimeout(third, 30);
})

Note: The pseudo-elements generated by ::before and ::after are contained by the element's formatting box, and thus don't apply to replaced elements such as img, or to br elements.

.container {
  display: flex;
  align-items: center;
}

<div class="btn btn1" data-num="1"></div>
<div class="btn btn2" data-num="2"></div>
<div class="block active" id="block1"></div>
<div class="block" id="block2"></div>