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The unsolvable z-index auto-increment.

cadienvan profile image Michael Di Prisco ・2 min read

The idea.

If you wrote my previous series CSS Only: to the infinity and beyond you know I like to play with CSS to explore its limits, and today I woke up with a strange, but simple idea: Auto-incremental z-indexes.

The development is pretty simple and straightforward: Being able to have a container div with many .layer divs inside of it, each one with an incremental z-index, auto-calculated via CSS.

I did many tries and I'll try to explain them in a simple and quick way.

TL;DR: It doesn't work. Or, at least, I didn't find a way to.

Let's go!

The HTML

The HTML part is the easiest one as it's exactly the one explained above:

<div class="main">
  <div class="layer">
    1
  </div>
  <div class="layer">
    2
  </div>
</div>
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The CSS - Part 1

I wanted to use some modern CSS and went for Custom Properties aka CSS Variables.

.main {
  --z-index: 0;
}

.layer {
  z-index: var(--z-index);
}

.layer + .layer {
  --z-index: var(--z-index) + 1;
}
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As soon as I wrote that, I remembered you can't do calcs like that in CSS. So I went for the calc() function.

The CSS - Part 2

.main {
  --z-index: 0;
}

.layer {
  z-index: var(--z-index);
}

.layer + .layer {
  --z-index: calc(var(--z-index) + 1);
}
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Buuuuut, it doesn't work either.
You can't calc things like that in CSS, so I had to find another way.

Eureka!

Have you ever read about CSS Lists And Counters? I did in the past, but I've never used them as I never had a chance to, so, I though, "Maybe I could give it a try!"

The CSS - Part 3

Let's go full counters then!

.main {
  counter-reset: section;
}

.layer {
  z-index: counter(section);
}

.layer + .layer {
  counter-increment: section;
}
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I started with a counter-reset and then counter-incremented it at every subsequent selector.

But the CSS counter() function returns a <string> and the z-index property is looking for an <integer>, so well.. It doesn't work.

Conclusion

I guess my dream will stay a dream for a little as I don't know any other possible CSS-only solution for this.

That's why I'm asking you to think about it and let me know if you can find a way to achieve my goal.

Can we do it with the current CSS specs? Will we ever be able to?

Let's #discuss it in the comments.

Wanna give it a try? Here's a Codepen you can fork.

Discussion (2)

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merri profile image
Vesa Piittinen

The layout elements are already autoincremental in z-index. z-index is an escape hatch when you have no other means to fix paint priority.

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cadienvan profile image
Michael Di Prisco Author

I know, it was just a test to try to deliberately separate layers in your layout. It was just a cool experiment :)