<!DOCTYPE html>
<html lang="ru">
<head>
  <title>z-index — Контекст наложения — Дока</title>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <style>
    *, *::before, *::after {
      margin: 0;
      padding: 0;
      box-sizing: border-box;
    }

    body {
      min-height: 100vh;
      padding: 50px;
      display: flex;
      align-items: center;
      justify-content: center;
      background-color: #18191C;
      color: #FFFF