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                <!-- test case: element with pseudo element -->

<!-- explanation (not test case) -->
  <pre><code>div::after {
  content: var(--mouse-x) ' / ' var(--mouse-y);
  <h1>If this worked...</h1>
    We should see something like this: <b><span class="x">245</span> / <span class="y">327</span></b> updating with the mousemove coordinates inside the pseudo <i>::after</i> element for the bubble. 


                /* what I want!! */
div::after {
  content: var(--mouse-x, 245) " / " var(--mouse-y, 327);

/* setup and presentation styles */
div::before {
  content: 'mouse position:';
div {
  position: absolute;
  top: 0;
  left: 0;
  transform: translate(calc(var(--mouse-x, 245) * 1px), calc(var(--mouse-y, 327) * 1px));
  width: 10em;
  height: 10em;
  background: #ff3b80;
  color: #fff;
  display: flex;
  flex-flow: column;
  align-items: center;
  justify-content: center;
  border-radius: 100%;
  will-change: transform;
body {
  margin: 2em;
  font-family: sans-serif;
p {
  max-width: 50%;
  min-width: 25em;
a {
  color: #0ebeff;
  text-decoration: none;


                document.addEventListener('mousemove', (e) => {'--mouse-x', e.clientX)'--mouse-y', e.clientY)
  // output for explanation text
  document.querySelector('.x').innerHTML = e.clientX
  document.querySelector('.y').innerHTML = e.clientY