#css-transitions

CSS @starting-style for entry animations with no JS

.toast {
  display: none;
  opacity: 0;
  translate: 0 12px;
  transition: opacity 0.2s ease, translate 0.2s ease, display 0.2s allow-discrete;

@starting-style supplies the missing before-state so a CSS transition can animate an element's first render — no class-added-next-frame trick, no library.