I’ve shared a way to add a pop-up or modal lightbox with CSS only on my earlier post, Easy Modal Lightbox Pop-Up. I’ve received many request how to make this modal pop up on page load instead of using a button. So, here’s how-to:

HTML

Copy the code below and paste into your HTML document.

<div id="modal-1" class="modal animate-opacity">
   <div class="modal-content">
      <div class="modal-inner">
         <span onclick="document.getElementById('modal-1').style.display='none'" class="modal-close">&times;</span>
         <h4>Modal Headline</h4>
         <p>Modal description goes here.</p>
      </div> 
   </div>
</div>

CSS

Copy the code below and paste into your CSS document.

.modal {
	z-index: 10;
	display: block;
	padding-top: 100px;
	position: fixed;
	left: 0;
	top: 0;
	width: 100%;
	height: 100%;
	overflow: auto;
	background-color: rgb(0,0,0);
	background-color: rgba(0,0,0,0.5)
}
.modal-content {
	margin: auto;
	background-color: #fff;
	position: relative;
	padding: 0;
	outline: 0;
	max-width: 600px
}
.modal-inner { padding: 20px 30px; }
.modal-close {
	-webkit-touch-callout: none;
	-webkit-user-select: none;
	-khtml-user-select: none;
	-moz-user-select: none;
	-ms-user-select: none;
	user-select: none;
	position: absolute;
	right: 0;
	top: 0;
	background: #ccc;
	padding: 6px 10px;
}
.animate-opacity { animation: opac 0.8s }@keyframes opac{from{opacity:0} to{opacity:1}}

.animate-opacity is optional as it’s the animated effect when modal opens.

You can tweak the animation time. It’s currently 0.8 seconds opac 0.8s , but change the timing as your need.