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Spriting images is a well-known, well-documented way to speed up page load times. The standard technique is to set a sprite sheet image as the background-image of an element, and position it so the proper sprite shows through. This works great for icons and other images that aren’t part of the actual content, but not so great for images that belong in the page content.

The problem

Images that are part of the content should be in img tags. This is correct semantically, good for accessibility, and good for interoperability. With the typical approach to spriting images, you would need empty container elements that have a background-image property. Those empty elements have no semantic meaning, and are worthless to screenreaders and other crawlers that may access your page. This is the situation I found myself in on Optimizely’s homepage when I sprited the customer logos and benefits graphics.

The solution

Solving this problem is actually fairly simple. First, you’ll need markup similar to the following:

<div class=”sprite-container sprite-track”>
<img src=”sprites.png” alt=”Graphic of tracking elements on a web page”>
</div>

View the Codepen

The .sprite-container element acts as a window that only lets one sprite show through. It has the same width and height as the sprite, and has overflow: hidden to prevent the other sprites from showing through.

Then the image needs to be positioned so the sprite we want shown is at the top left corner of the window, which is accomplished by setting position: absolute and the top and left properties correctly. SCSS:

.sprite-container {
display: inline-block;  // or block
height: 275px;          // size of the image we want to show
width: 400px;
overflow: hidden;       // prevent the other sprites from showing
position: relative;     // so the image can be positioned absolutely

img {
position: absolute;
}

&.sprite-track img {
left: 0;
top: -210px;
}

// other sprites
}

Codepen:

Generating this CSS can be a pain, but can be accomplished with Compass.

Closing Thoughts

This method lets us use sprites in img tags, which is more semantic than empty divs or spans (depending on the use case). However, it has a couple of drawbacks. First, it requires a container element around your image. This could add unneccessary bloat to your markup (in my case the image already had a parent element). Second, and greatest of all, the image src attribute now references an entire sprite sheet, when technically it should only reference the actual image that it’s showing. Screenreaders won’t care at all, but some services may use this image for other purposes. For example, Facebook grabs a representative image of your page when a link is shared, and it could grab a sprite sheet instead of the actual image. That’s no good.

Ultimately it’s a tradeoff between having a container element and actual img tag in your markup versus an empty element whose background image is a sprite. Overall, I think the pros (semantic HTML; better accessibility) outweigh the cons (container element; src attribute is a sprite sheet), but this cost-benefit analysis can be done quantitatively in an AB testing tool. If you want to use sprites with actual img tags, this is a solid technique. If anyone has any thoughts on how to overcome the drawbacks, let me know @jlzych.

In a follow up article, I discuss how to automate this with Compass.

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