Do Browsers Download Unused CSS Images?

Unused CSS Images: Downloaded or Ignored?

When CSS rules reference images that are not applied to any elements, it raises the question of whether browsers download these unused images or discard them altogether. Let's delve into the answer.

Browser-Dependent Behavior

Determining whether unused CSS images are downloaded depends on the browser implementation of the CSS specification. A quick experiment reveals the following behavior across popular browsers:

• Chrome, Firefox, Safari, IE8, and IE7: Do not download unused CSS images.
• IE6: Behavior is unknown and requires further testing.

Implementation Details

Browsers that ignore unused CSS images likely follow the CSS 2.1 specification, which states that "If a property set has no effect on the style of an element, then the property set has no effect on the computation of that element." In other words, if an image is not applied to an element, it is treated as having no effect on the element's style and is not downloaded.

Implications for Performance

The behavior of unused CSS images can have implications for website performance. While the amount of data in unused images may not be significant, it can still contribute to overall page load time, especially if there are numerous unused images. By using a tool like CSS Compressor to remove unused CSS styles, you can minimize the number of images that are downloaded, improving website performance.

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