JSFiddle - React, Tailwind, and code Playground

HTML

<div class="priority-cell">

</div>
<div class="priority-cell">

</div>
<div class="priority-cell">

</div>
<div class="priority-cell">

</div>

CSS

.priority-cell {
  background: #0f0;
  border: 1px solid #000;
  height: 250px;
  width: 250px;
}

JavaScript

var pcells = document.getElementsByClassName('priority-cell');

for (var i=0; i < pcells.length; i++) {
  pcells[i].onclick = function (e) {
    var div = document.createElement('div');
       div.style.backgroundColor = "red";
       div.style.height = "10px";
       div.style.width = "10px";

       e.target.appendChild(div);
  };
}