HTML

html
 
 
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<!-- Button to open the modal -->
<button id="myBtn">Open Modal</button>

<!-- The Modal -->
<div id="myModal" class="modal">
  <!-- Modal content -->
  <div class="modal-content">
    <span class="close">&times;</span>
    <p>Modal content goes here.</p>
  </div>
</div>

CSS

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

/* Modal Content/Box */
.modal-content {
  background-color: #fefefe;
  margin: 15% auto; /* 15% from the top and centered */
  padding: 20px;
  border: 1px solid #888;
  width: 80%; /* Could be more or less, depending on screen size */
}

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

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

JavaScript

javascript
 
 
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// Get the modal
var modal = document.getElementById("myModal");

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

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

// When the user clicks on 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";
  }
}

HTML:

html
 
 
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<div id="modalWindow" style="display: none;">
  <div>
    <div>
      <p>Modal content here.</p>
    </div>
    <div>
      <input type="button" value="Close" onclick="hideModal()" />
    </div>
  </div>
</div>

CSS:

css
 
 
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#modalWindow {
  position: fixed;
  top: 0;
  bottom: 0;
  left: 0;
  right: 0;
  background: rgba(0, 0, 0, 0.7);
  margin: auto;
  padding: 10px;
  width: 500px;
  font-family: arial;
  font-size: 14px;
  text-align: center;
}

JavaScript:

javascript
 
 
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// reads the DOM element
let modalWindow = document.querySelector('#modalWindow');

// shows the modal window
function showModal() {
  modalWindow.style.display = 'block';
}

// hides the modal window
function hideModal() {
  modalWindow.style.display = 'none';
}

Creating a centered modal is possible in HTML and CSS. To do so, a modal element should have a container div with display: flex and justify-content: center. Additionally, the position must be set as fixed to allow the modal to stay in a static position on the page.

For example, HTML markup may look something like:

html
 
 
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<div class='modal-container'>
  <div class='modal'>
    <!-- Modal content -->
   </div>
</div> 

CSS may look something like:

css
 
 
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.modal-container {
  position: fixed;
  display: flex;
  justify-content: center;
  align-items: center;
}

.modal {
  width: 500px;
  height: 400px;
  position: relative;
}

Further reading on creating centered modals: Centering a Modal with CSS Flexbox.

To create a centered modal, you can use the following CSS code:

css
 
 
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.modal {
  position: fixed;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

This code will set the modal's position to fixed and center it vertically and horizontally within its container. To learn more, you can check out this video by Traversy Media.