JSFiddle - React, Tailwind, and code Playground

HTML

<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="stylesheet" href="/style.css">
  </head>
<body>
  <div>
    <button onclick="show('popup1')">Show with @starting-style</button>
    <button class="irrelevant popup popup1" onclick="hide('popup1')">Close 1</button>
  </div>
  <div>
    <button onclick="show('popup2')">Show without @starting-style</button>
    <button class="irrelevant popup popup2" onclick="hide('popup2')">Close 2</button>
  </div>
</body>
</html>

CSS

* {
  padding: 0;
  margin: 0;
}

body {
  display: grid;
  grid-template-columns: 50vw 50vw;
  gap: 1rem;
  padding: 1rem;
}

body > div {
  display: flex;
  flex-direction: column;
  border: 3px solid black;
  height: 100vh;
}

.irrelevant {
  height: 200px;
  background: darkgray;
}

.popup {
  display: none;
  transition:
    display 0.7s,
    translate 0.7s ease-out;
  translate: 0 50vh;
}

.popup.open {
  display: block;
  translate: 0 0;
}

.popup1.open {
  @starting-style {
    translate: 0 50vh;
  }
}

JavaScript

function getElement(popupName) {
  return document.querySelector("." + popupName)
}

function hide(popupName) {
  getElement(popupName).classList.remove("open")
}

function show(popupName) {
  getElement(popupName).classList.add("open")
}