"Popup" in the same window!

@ytregnn asked for an in-window “popup” (think AJAX/modal). Several replies pointed to ready-made lightbox-style scripts (@knowmore, @ghostsquad, @Phaelax). Below is a minimal, modern pattern you can use instead of a third-party library — it degrades gracefully (links still work without JS), is responsive, and includes basic accessibility and close controls.

<!-- thumbnail link keeps href as fallback -->
<a href="images/large1.jpg" class="thumb" data-large="images/large1.jpg">
  <img src="images/thumb1.jpg" alt="Photo caption">
</a>

<!-- modal (hidden by default) -->
<div id="modal" class="modal" role="dialog" aria-hidden="true" aria-label="Image preview">
  <button id="modalClose" class="modal-close" aria-label="Close">Close</button>
  <img id="modalImg" src="" alt="">
</div>

<style>
.modal { display:none; position:fixed; inset:0; background:rgba(0,0,0,.85);
        justify-content:center; align-items:center; z-index:9999; }
.modal.open { display:flex; }
.modal img { max-width:90vw; max-height:90vh; }
body.noscroll { overflow:hidden; }
</style>

<script>
document.addEventListener('click', function(e){
  var t = e.target.closest && e.target.closest('.thumb');
  if (!t) return;
  e.preventDefault();
  var src = t.dataset.large || t.href;
  var m = document.getElementById('modal');
  document.getElementById('modalImg').src = src;
  m.classList.add('open'); m.setAttribute('aria-hidden','false');
  document.body.classList.add('noscroll');
  document.getElementById('modalClose').focus();
});
function close(){ var m=document.getElementById('modal'); m.classList.remove('open');
  m.setAttribute('aria-hidden','true'); document.body.classList.remove('noscroll');
  document.getElementById('modalImg').src=''; }
document.getElementById('modalClose').addEventListener('click', close);
document.addEventListener('keydown', function(e){ if (e.key==='Escape') close(); });
</script>

Notes and quick tips: keep the anchor href as a no-JS fallback; use data- attributes to avoid loading full images until needed; lock body scrolling while the modal is open; add error handling for failed image loads; use event delegation for performance on galleries; consider history.pushState if you need deep-linking. If you prefer turnkey behavior, the earlier replies list ready-made options, but the tiny vanilla approach above avoids extra dependencies and common conflicts.