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CSS

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <dialog id="dialog">
  <form method="dialog">
    <p>Hi, I'm a dialog.</p>
    <button>OK</button>
  </form>
</dialog>

<button onclick="dialog.showModal()">Open Dialog</button>
!

CSS 

              
                dialog {
  /* exit stage position of the dialog */
  transform: translateY(-20px);
  
  /* off stage opacity and default transition styles */
  /* allow-discrete is a `transition-behavior` */
  &, &::backdrop {
    transition: 
      /* time display none/block with the transitions */
      display 1s allow-discrete,
      /* time top-layer projection with the transitions */
      overlay 1s allow-discrete,
      /* transition opacity */
      opacity 1s,
      /* transition transforms */
      transform 1s;

    /* both dialog and it's backdrop are opacity 0 */
    /* when not showing */
    opacity: 0;
  }
  
  /* on stage styles */
  /* apply when dialog is shown */
  &[open] {
    opacity: 1;
    transform: translateY(0px);
    
    &::backdrop {
      opacity: 1;
    }
  }
  
  /* entering stage styles */
  /* used on first render when dialog has been */
  /* cloned and projected into the top-layer */
  @starting-style {
    &[open], 
    &[open]::backdrop {
      opacity: 0;
    }
    
    /* specific transform for dialog and not the backdrop */
    /* enter stage from 20px below the on stage resting style */
    &[open] {
      transform: translateY(20px);
    }
  }
}

::backdrop {
  background-image: 
    radial-gradient(
      circle at 1px 1px, 
      light-dark(#0007, #fff1) 1px, 
      #000c 0
    );
  background-size: 20px 20px;
}

html {
  block-size: 100%;
  color-scheme: dark light;
}

body {
  min-block-size: 100%;
  font-family: system-ui, sans-serif;
  
  display: grid;
  place-content: center;
  margin: 0;
}
!

JS
!
999px

Console