Pseudo-Randomly Adding Illustrations with CSS

Avatar of Robin Rendle
Robin Rendle on (Updated on )

Take the pain out of building site search with the Algolia hosted API. Start free now!

Between each post of Eric Meyer’s blog there’s this rather lovely illustration that can randomly be one of these five options:

Eric made each illustration into a separate background image then switches out that image with the nth-of-type CSS property, like this:

.entry:nth-of-type(2n+1)::before {
   background-image: url(image-1.png);

.entry:nth-of-type(3n+1)::before {
   background-image: url(image-2.png);

.entry:nth-of-type(4n+1)::before {
   background-image: url(image-3.png);

.entry:nth-of-type(5n+1)::before {
   background-image: url(image-4.png);

This seems like a good time to plug our very own little :nth Tester tool. It definitely helps me understand what something like (2n+1) means in English. You can type in any string you like and see what effect that has on your site:

Anyway, back to Eric’s post. As he mentions, his technique is pseudo-random in that it looks like a random image on the page but it technically isn’t. Either way, I think it’s a really lovely technique! And it certainly breaks up the visual monotony that happens when you’re looking at a website for too long.

Here’s what it looks like in practice:

Lovely stuff!

Another way to do this is to use random numbers in CSS. For example, we could set a variable in JavaScript and then apply it with CSS custom properties. Or we could put all the images in a single sprite file and change the background-position based on a random number.

This is definitely one of those things in CSS where there are no wrong answers; just different ways to do the same awesome thing!