Bootstrap 4 Ya

Ejemplo
Vista Previa

<!doctype html>
<html>
<head>
  <title>Prueba de Bootstrap 4</title> 
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
  <link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous">
</head>
<body>
    <div class="container"> 
        <div class="alert alert-danger alert-dismissible mt-5">
            <p>El sitio permanecerá cerrado toda la semana</p>
            <button type="button" class="close" data-dismiss="alert">
              <span>x</span>
            </button>
          </div>      
    </div>