Question

There are two CSS files referenced on the same page: A generic.css file and a custom.css file. The generic file has default styles in it that are overridden by the custom.css file for the same elements. This allows users of the site to customize or “skin” their pages without needing to recreate the entire generic.css file. Only a few styles would be overridden.

My question is the following: If the generic.css file has a style for an element with a background image and that same style is overridden in the custom.css with a different background image, is the first image ever downloaded by the browser?

Also, I want to find out if this is bad practice – customizing or “skinning” a generic CSS file with another custom CSS file to override a few styles, including specifying different background images.

Answer 1

While not totally definitive, this site ran some tests regarding this. The significant statement from that site that is related to your question is:

CSS images are kicked off not in the order in which they appear in the
CSS but in the order in which they’re called in the HTML. I’m not sure
of the precise moment when the download is initiated, but my guess is
that it happens just after the CSS rules have been matched, when the
property values are assigned to the DOM elements.

This at least tentatively confirms what I thought I remembered in the back of my mind reading on this a few years back, namely, that background images which are not ever displayed (as in the generic.css images being overridden by the custom.css images) are not ever downloaded.

Further confirmation of this would be the typical image preloader script that used to be so common prior to sprite images (and is still found in certain uses). It was designed to download images that would be used on :hover in css, because without it, the image would not load until the first hover was initiated, and this caused an unsightly delay. So that, also, argues for the fact that unless actually displayed (or preloaded), the background images themselves are never loaded.

I don’t think there are generally any issues with “skinning,” unless you are essentially overwriting most or all of the generic.css with custom.css, then one could argue, why load the generic at all. But as you said, normally there are just a few styles overwritten.