create an empty modal box on the current page and below is the ajax call you can see how to fetch the content in result from another html page.
$.ajax({url: "registration.html", success: function(result){
//alert("success"+result);
$("#contentBody").html(result);
$("#myModal").modal('show');
}});
once the call is done you will get the content of the page by the result to then you can insert the code in you modal's content id using.
You can call controller and get the page content and you can show that in your modal.
below is the example of Bootstrap 3 modal in that we are loading content from registration.html page...
index.html
------------------------------------------------
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js"></script>
<script type="text/javascript">
function loadme(){
//alert("loadig");
$.ajax({url: "registration.html", success: function(result){
//alert("success"+result);
$("#contentBody").html(result);
$("#myModal").modal('show');
}});
}
</script>
</head>
<body>
<!-- Trigger the modal with a button -->
<button type="button" class="btn btn-info btn-lg" onclick="loadme()">Load me</button>
<!-- Modal -->
<div id="myModal" class="modal fade" role="dialog">
<div class="modal-dialog">
<!-- Modal content-->
<div class="modal-content" >
<div class="modal-header">
<button type="button" class="close" data-dismiss="modal">×</button>
<h4 class="modal-title">Modal Header</h4>
</div>
<div class="modal-body" id="contentBody">
</div>
<div class="modal-footer">
<button type="button" class="btn btn-default" data-dismiss="modal">Close</button>
</div>
</div>
</div>
</div>
</body>
</html>
registration.html
--------------------
<!DOCTYPE html>
<html>
<style>
body {font-family: Arial, Helvetica, sans-serif;}
form {
border: 3px solid #f1f1f1;
font-family: Arial;
}
.container {
padding: 20px;
background-color: #f1f1f1;
width: 560px;
}
input[type=text], input[type=submit] {
width: 100%;
padding: 12px;
margin: 8px 0;
display: inline-block;
border: 1px solid #ccc;
box-sizing: border-box;
}
input[type=checkbox] {
margin-top: 16px;
}
input[type=submit] {
background-color: #4CAF50;
color: white;
border: none;
}
input[type=submit]:hover {
opacity: 0.8;
}
</style>
<body>
<h2>CSS Newsletter</h2>
<form action="/action_page.php">
<div class="container">
<h2>Subscribe to our Newsletter</h2>
<p>Lorem ipsum text about why you should subscribe to our newsletter blabla. Lorem ipsum text about why you should subscribe to our newsletter blabla.</p>
</div>
<div class="container" style="background-color:white">
<input type="text" placeholder="Name" name="name" required>
<input type="text" placeholder="Email address" name="mail" required>
<label>
<input type="checkbox" checked="checked" name="subscribe"> Daily Newsletter
</label>
</div>
<div class="container">
<input type="submit" value="Subscribe">
</div>
</form>
</body>
</html>