JSFiddle - React, Tailwind, and code Playground

HTML

<dialog open>
  <p>Greetings, one and all!</p>
  <form method="dialog">
    <button>OK</button>
  </form>
</dialog>

<p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p><p>Test</p>

CSS

body:has(dialog[open]) {
    overflow: hidden
  }

  body:has(dialog[open])::before {
    content: "";
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    backdrop-filter: blur(4px);
    background-color: rgb(0, 0, 0, 0.5);
  }