如何在另一个页面中显示弹出内容？

function openPopup() {
  var popupWindow = window.open("popup.html", "_blank", "width=400,height=300");
  popupWindow.document.write("<h1>This is a popup window</h1>");
}

<!-- HTML -->
<button onclick="openModal()">Open Modal</button>
<div id="myModal" class="modal">
  <div class="modal-content">
    <span class="close" onclick="closeModal()">&times;</span>
    <h1>This is a modal</h1>
  </div>
</div>

/* CSS */
.modal {
  display: none;
  position: fixed;
  z-index: 1;
  left: 0;
  top: 0;
  width: 100%;
  height: 100%;
  overflow: auto;
  background-color: rgba(0, 0, 0, 0.4);
}
.modal-content {
  background-color: #fefefe;
  margin: 15% auto;
  padding: 20px;
  border: 1px solid #888;
  width: 80%;
}
.close {
  color: #aaa;
  float: right;
  font-size: 28px;
  font-weight: bold;
}
.close:hover,
.close:focus {
  color: black;
  text-decoration: none;
  cursor: pointer;
}

// JavaScript
function openModal() {
  document.getElementById("myModal").style.display = "block";
}
function closeModal() {
  document.getElementById("myModal").style.display = "none";
}

<!-- HTML -->
<a href="image.jpg" class="popup-link">Open Image</a>

// JavaScript
$('.popup-link').magnificPopup({
  type: 'image'
});