CSS: backdrop

backdrop
<!DOCTYPE html>
<html>
 <head>
 <meta charset="utf-8">
 <title>::backdrop</title>
 <style>
  dialog {
  width: 300px;
  }
  dialog::backdrop {
  background: rgba(0,0,0,0.7);
  }
 </style>
 </head> 
 <body> 
 <button id="openDialog">Вход на сайт</button> 
 <dialog>
  <form id="authentication" action>
  <p><input name="login" required autofocus placeholder="Логин"></p>
  <p><input type="password" name="pass" required placeholder="Пароль"></p>
  <p><button type="submit">Войти</button> 
  <button id="clodeDialog">Закрыть окно</button></p>
  </form> 
 </dialog>
 <script>
  var dialog = document.querySelector('dialog');
  document.querySelector('#openDialog').onclick = function() {
  dialog.showModal();
  }
  document.querySelector('#clodeDialog').onclick = function() {
  dialog.close();
  }
 </script>
 </body> 
</html>