<dialog>
  <p>Hello! This is a dialog with an entry animation.</p>
  <form method="dialog">
    <button>OK</button>
  </form>
</dialog>

<button onclick="document.querySelector('dialog').showModal()">Open Dialog</button>
dialog[open] {
  translate: 0 0;
  transition: translate 0.7s ease-out, display 0.7s ease-out allow-discrete;
  
  @starting-style {
    translate: 0 100vh;
  }
}

/* Etc */

body,
button {
  font-family: system-ui;
  font-size: 1rem;
}

button {
  padding: 1rem;
  background: deeppink;
  color: white;
  font-weight: 600;
  border-radius: 3rem;
  border: none;
}

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.