I recently added to my growing library of image-preloading methods with a few new-&-improved techniques. After posting that recent preloading article, an even better way of preloading images using pure CSS3 hit me:

.preload-images {
	background: url(image-01.png) no-repeat -9999px -9999px;
	background: url(image-01.png) no-repeat -9999px -9999px,
		    url(image-02.png) no-repeat -9999px -9999px,
		    url(image-03.png) no-repeat -9999px -9999px,
		    url(image-04.png) no-repeat -9999px -9999px,
		    url(image-05.png) no-repeat -9999px -9999px;
	}

Using CSS3’s new support for multiple background images, we can use a single, existing element to preload all of the required images. Compare this method with the old way of using CSS to preload images:

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Preloading images is a great way to improve the user experience. When images are preloaded in the browser, the visitor can surf around your site and enjoy extremely faster loading times. This is especially beneficial for photo galleries and other image-heavy sites where you want to deliver the goods as quickly and seamlessly as possible. Preloading images definitely helps users without broadband enjoy a better experience when viewing your content. In this article, we’ll explore three different preloading techniques to enhance the performance and usability of your site.

Method 1: Preloading with CSS and JavaScript

There are many ways to preload images, including methods that rely on CSS, JavaScript, and various combinations thereof. As one of my favorite topics here at Perishable Press, I have covered image preloading numerous times:

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I have written previously on the fine art of preloading images without JavaScript using only CSS. These caching techniques have evolved in terms of effectiveness and accuracy, but may be improved further to allow for greater cross-browser functionality. In this post, I share a “CSS-only” preloading method that works better under a broader set of conditions.

Previous image-preloading techniques target all browsers, devices, and media types. Unfortunately, certain browsers do not load images that are hidden directly (via the <img> element) or indirectly (e.g., via the parent <div> element) using either display:none; or visibility:hidden;. Further problematic is the potential unintentional display of images on pages when presented via specifically designed print stylesheet.

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Responding to my first attempt at preloading images without JavaScript, CSS-Guru David Bowman graces his audience with a most enlightening triage of comments.

Apparently, the image-preloading technique explained in the article is “major overkill” and “totally ridiculous.” Of course, I will be the first to admit that I am no expert in CSS, but I do enjoy sharing my discoveries and watching as people improve upon them. My first attempt at preloading images without JavaScript may indeed be “pretty crappy,” but it certainly works.

Fortunately, several weeks prior to Mr. Bowman’s dazzling performance, insightful reader Duarte helps the community by sharing a far more elegant solution using display: none;. Here is an example of its implementation:

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After reading my previous article on preloading images without JavaScript ¹, Nanda pointed out that adding extra markup to preload images is not the best approach, especially where Web Standards are concerned. Mobile devices, for example, may experience problems when dealing with the following preloading technique:

/* ADD THIS TO CSS */
div#preloaded-images {
   position: absolute;
   overflow: hidden;
   left: -9999px; 
   top: -9999px;
   height: 1px;
   width: 1px;
}

<!-- ADD THIS TO XHTML -->
<div id="preloaded-images">
   <img src="http://perishablepress.com/image-01.png" width="1" height="1" alt="Image 01" />
   <img src="http://perishablepress.com/image-02.png" width="1" height="1" alt="Image 02" />
   <img src="http://perishablepress.com/image-03.png" width="1" height="1" alt="Image 03" />
</div>

Thus, as Nanda suggests, it is better to preload images using only CSS. Using the CSS background property, we can preload images via existing <div>s, <span>s, or other elements in the (X)HTML markup.

Let’s say you have three images (e.g., image_01.png, image_02.png, and image_03.png) that you would like to preload using this method. First, examine your markup for three identifiable <div>s (or other elements) that may be used as CSS hooks for the preloaded images. For example, looking at the source code of the current page, I would choose the following three divisions:

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Clean, easy, effective. You don’t need no stinking JavaScript to preload your images. Nope. Try some tasty CSS and (X)HTML instead! Here’s how.. (only two steps!)

Step 1 — Place this in your CSS file:

div#preloaded-images {
   position: absolute;
   overflow: hidden;
   left: -9999px; 
   top: -9999px;
   height: 1px;
   width: 1px;
}

Step 2 — Place this at the bottom of your (X)HTML document:

<div id="preloaded-images">
   <img src="http://perishablepress.com/image-01.png" width="1" height="1" alt="" />
   <img src="http://perishablepress.com/image-02.png" width="1" height="1" alt="" />
   <img src="http://perishablepress.com/image-03.png" width="1" height="1" alt="" />
</div>

..and that’s a wrap! All images are preloaded and ready for calling as you please. Completely valid, standards-compliant image preloading via CSS and (X)HTML!!

Fast-loading pages reduce errors, conserve bandwidth, and please visitors. One way to decrease loading times and enhance performance involves maximizing image display efficiency. Your mantra for achieving image efficiency should be "reuse, optimize, and preload.". While each of these methods plays an important role, this article will focus on methods for preloading images. Consult your server error logs to identify web pages that may require image help. Note: preloading images does not reduce bandwidth! It only decreases apparent load time, thereby enhancing user experience.

Preloading via the CSS Display Property

This method is a very common method for preloading images via the CSS display: none; directive. To use this method, first add the following code to your CSS rules:

<style type="text/css">
<!--/*--><![CDATA[/*><!--*/
   img.preload { display: none; }
/*]]>*/-->
</style>

Then, add the preload class to each image tag that should be preloaded:

<img src="image1.jpg" alt="Image Caption 1" height="33" width="33" class="preload" />
<img src="image2.jpg" alt="Image Caption 2" height="33" width="33" class="preload" />
<img src="image3.jpg" alt="Image Caption 3" height="33" width="33" class="preload" />
<img src="image4.jpg" alt="Image Caption 4" height="33" width="33" class="preload" />

Include the array of preloading image elements to the bottom of the home page, just before the body tag. Placing the images at the bottom of the page requires the browser to load the entire page before preloading any of the images. Finally, simply use the same image path and name when referencing the preloaded images on subsequent pages. The browser will have cached the images and will be able to load them instantly. Remember not to use the preload class for images that are meant to be displayed.

Preloading Tuf with JavaScript

This method uses JavaScript to cache specified images as the document is loaded. Images will then be displayed immediately as they are called. The JavaScript itself degrades peacefully and has no effect on browsers that do not support JavaScript. The script is as follows. Simply replace ../path/to/image-0n.gif with the corresponding path and name of the image to be preloaded. Then, simply call the images as normal (e.g., <img src="../path/to/image-0n.gif" alt="Image Caption" />) wherever they are required.

<script>
<!--//--><![CDATA[//><!--

if (document.images) {

	img1 = new Image();
	img2 = new Image();
	img3 = new Image();

	img1.src = "../path/to/image-01.gif";
	img2.src = "../path/to/image-02.gif";
	img3.src = "../path/to/image-03.gif";

}

//--><!]]>
</script>

You can also wrap this method in a function like so:

function preloader() {
	if (document.images) {

		var img1 = new Image();
		var img2 = new Image();
		var img3 = new Image();

		img1.src = "../path/to/image-01.gif";
		img1.src = "../path/to/image-02.gif";
		img1.src = "../path/to/image-03.gif";
	}
}