popup.html

<button onclick="openTab()">Open</button>

<div id="popup">
<p>anything in here is in the popup</p>
</div>

<script>
function openTab() {
var popup = document.getElementById("popup")
popup.classList.toggle("open")
}
</script>

<style>
#popup {
display: none;
}

.open {
display: initial !important;
}
</style>