Try to match id instead of whole object like,

$(window).on('click', function(event){
    if(event.target.id == 'myModal'){
        $('#myModal').css({display: "none"});
    }
});

// Get the button that opens the modal
var btn = document.getElementById("myBtn");
var modal = document.getElementById('myModal');

// Get the <span> element that closes the modal
var span = document.getElementsByClassName("close")[0];

// When the user clicks the button, open the modal
btn.onclick = function() {
    modal.style.display = "block";
}

// When the user clicks on <span> (x), close the modal
span.onclick = function() {
    modal.style.display = "none";
}

// When the user clicks anywhere outside of the modal, close it
/*window.onclick = function(event) {
    if (event.target == modal) {
        modal.style.display = "none";
    }
}*/
$(window).on('click', function(event){
    if(event.target.id == 'myModal'){
        $('#myModal').css({display: "none"});
    }
});

/* The Modal (background) */
.modal {
    display: none; /* Hidden by default */
    position: fixed; /* Stay in place */
    z-index: 1; /* Sit on top */
    padding-top: 100px; /* Location of the box */
    left: 0;
    top: 0;
    width: 100%; /* Full width */
    height: 100%; /* Full height */
    overflow: auto; /* Enable scroll if needed */
    background-color: rgb(0,0,0); /* Fallback color */
    background-color: rgba(0,0,0,0.4); /* Black w/ opacity */
}

/* Modal Content */
.modal-content {
    position: relative;
    background-color: #fefefe;
    margin: auto;
    padding: 0;
    border: 1px solid #888;
    width: 80%;
    box-shadow: 0 4px 8px 0 rgba(0,0,0,0.2),0 6px 20px 0 rgba(0,0,0,0.19);
    -webkit-animation-name: animatetop;
    -webkit-animation-duration: 0.4s;
    animation-name: animatetop;
    animation-duration: 0.4s
}

/* Add Animation */
@-webkit-keyframes animatetop {
    from {top:-300px; opacity:0}
    to {top:0; opacity:1}
}

@keyframes animatetop {
    from {top:-300px; opacity:0}
    to {top:0; opacity:1}
}

/* The Close Button */
.close {
    color: white;
    float: right;
    font-size: 28px;
    font-weight: bold;
}

.close:hover,
.close:focus {
    color: #000;
    text-decoration: none;
    cursor: pointer;
}

.modal-header {
    padding: 2px 16px;
    background-color: #5cb85c;
    color: white;
}

.modal-body {padding: 2px 16px;}

.modal-footer {
    padding: 2px 16px;
    background-color: #5cb85c;
    color: white;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>

<h2>Animated Modal with Header and Footer</h2>

<!-- Trigger/Open The Modal -->
<button id="myBtn">Open Modal</button>
<div id="myModal" class="modal">

  <!-- Modal content -->
  <div class="modal-content">
    <div class="modal-header">
      <span class="close">×</span>
      <h2>Modal Header</h2>
    </div>
    <div class="modal-body">
      <p>Some text in the Modal Body</p>
      <p>Some other text...</p>
    </div>
    <div class="modal-footer">
      <h3>Modal Footer</h3>
    </div>
  </div>

</div>

url is not one of the options in jQuery UI dialog.

One of the things that has worked for me is to have an iframe inside the div that is your dialog, and set its url property on the open event.

Like:

<div id="dialog">
    <iframe id="myIframe" src=""></iframe>
</div>
<button id="dialogBtn">Open Dialog</button>

And JS:

$("#dialog").dialog({
    autoOpen: false,
    modal: true,
    height: 600,
    open: function(ev, ui){
             $('#myIframe').attr('src','http://www.jQuery.com');
          }
});

$('#dialogBtn').click(function(){
    $('#dialog').dialog('open');
});

You would find that you need some styling on the iframe to get it look nice, though.

#myIframe{
  height: 580px;
}

EDIT: Working version - http://jsbin.com/uruyob/1/edit

Try using window.onload instead of onclick, following the w3schools tutorial change this

img.onclick = function(){
  modal.style.display = "block";
  modalImg.src = this.src;
  captionText.innerHTML = this.alt;
}

to this:

window.onload = function(){
  modal.style.display = "block";
  modalImg.src = img.src;
  captionText.innerHTML = img.alt;
}