title: Why when my modal opens, the background is darkened?
body: <p>I have an example of my modal code <a href="https://www.w3schools.com/howto/tryit.asp?filename=tryhow_css_modal" rel="nofollow noreferrer">here</a>. </p>
  
  <p>I can not figure out which part of the code makes the background darkened when the modal is open, please give me any hint, thank you.</p>