inserting custom css, html and js into the current page as a popup window

Question

I found a small solution BUT its not the best instead of what you ask!

You can create an custom page template in your theme or child theme https://www.wpbeginner.com/wp-themes/how-to-create-a-custom-page-in-wordpress/#manual

And paste the code:

https://www.w3schools.com/howto/howto_js_alert.asp

<!DOCTYPE html>
<html>
<head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
.alert {
  padding: 20px;
  background-color: #f44336;
  color: white;
}

.closebtn {
  margin-left: 15px;
  color: white;
  font-weight: bold;
  float: right;
  font-size: 22px;
  line-height: 20px;
  cursor: pointer;
  transition: 0.3s;
}

.closebtn:hover {
  color: black;
}
</style>
</head>
<body>

<h2>Alert Messages</h2>

<p>Click on the "x" symbol to close the alert message.</p>
<div class="alert">
  <span class="closebtn" onclick="this.parentElement.style.display='none';">&times;</span> 
  <strong>Danger!</strong> Indicates a dangerous or potentially negative action.
</div>

</body>
</html>