Basic Modal Popup

<!-- HTML structure for modal -->
<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close-button">&times;</span>
    <p>Some text in the Modal..</p>
  </div>
</div>
<button id="openModal">Open Modal</button>

/* CSS for modal */
.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 */
  background-color: rgba(0, 0, 0, 0.5); /* Black w/ opacity */
}

.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 {
  color: #aaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
}

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

// JavaScript to handle modal popup
// Get the modal
var modal = document.getElementById('myModal');

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

// Get the element that closes the modal
var span = document.getElementsByClassName('close-button')[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';
  }
}