<div id="popup-container">
  <div id="popup-header">Hello World</div>
  <div id="popup-content">This is a simple popup.</div>
  <button id="close-btn">Close</button>
</div>

#popup-container {
  position: fixed;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
  background-color: #fff;
  padding: 20px;
  width: 300px;
  border-radius: 5px;
  box-shadow: 0px 0px 10px rgba(0, 0, 0, 0.5);
}
 
#popup-header {
  font-size: 24px;
  font-weight: bold;
  margin-bottom: 10px;
}
 
#popup-content {
  font-size: 16px;
  line-height: 1.5;
  margin-bottom: 20px;
}
 
#close-btn {
  display: block;
  margin: 0 auto;
  padding: 10px 20px;
  background-color: #333;
  color: #fff;
  border-radius: 5px;
  cursor: pointer;
}

var popupContainer = document.getElementById("popup-container");
var closeBtn = document.getElementById("close-btn");
 
function openPopup() {
  popupContainer.style.display = "block";
}
 
function closePopup() {
  popupContainer.style.display = "none";
}
 
closeBtn.addEventListener("click", closePopup);
 
window.onload = function() {
  openPopup();
};