<div class="zk-test"><p data-zk="cil">JS neproběhl</p><button type="button" data-zk="tl">Klikni</button></div>

.zk-test { padding: 1rem; border: 2px solid #16A34A; border-radius: 8px; }
.zk-test p { font-weight: 700; }

document.addEventListener('DOMContentLoaded', function () {
  var cil = document.querySelector('[data-zk="cil"]');
  var tl  = document.querySelector('[data-zk="tl"]');
  if (!cil || !tl) return;
  cil.textContent = 'JS probehl pri nacteni';
  tl.addEventListener('click', function () {
    cil.textContent = 'Kliknuti funguje: ' + new Date().getSeconds();
  });
});