Example BEM module

//Block
.notification-popup {
  width:100%;
  height:100px;
}

//Element
.notification-popup_content {
   color:black;
}

//Element child
.notification-popup_content_closebtn {
  width:50px;
  height:50px;
}

//Modifier
.notification-popup--error {
   @extend .notifcation-popup;
   border: 1px solid red
   background-color: pink;
   color:#fff;
}

<!-- example -->
<div class="notification-popup">
  Content
  <div class="notification-popup_content">
    <div class="notification-popup_content_closebtn"></div>
      Lovely content all goes in here
  </div>
</div>