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While working on something else entirely¹, I had a random thought:

Could the :checked and and :has pseudo-classes and the subsequent-sibling (~) selector be combined to perform interactive filtering without JavaScript?

Turns out, yes. Have a play with the filters on the side of this. You can either use:

• “OR” mode, so you can show e.g. “all mammals and carnivores”, or
• “AND” mode, so you can show e.g. “all mammals that are carnivores”.

Filter the animals!

(if it doesn’t work right where you are, e.g. in a feed reader, you can view it “standalone”)

The source code is available to download under the Unlicense, but the animal images are CC-BY licensed (with thanks to Aslan Almukhambetov).

How does it work?

There’s nothing particularly complicated here, although a few of the selectors are a little verbose.

First, we set the initial state of each animal. In “OR” mode, they’re hidden, because each selected checkbox is additive. In “AND” mode, they’re shown, because checking a checkbox can only ever remove an animal from the result set:

#filters:has(#filter-or:checked) ~ #animals .animal {
  display: none;
}

#filters:has(#filter-and:checked) ~ #animals .animal {
  display: flex;
}

The magic of the :has pseudo-class is that it doesn’t change the scope, which means that after checking whether “AND” or “OR” is checked within the #filters, the #animals container is still an adjacent element.

Then all we need to do is to use daisy-chain :has to show animals with a particular class if that class is checked in “OR” mode, or to hide animals that don’t have a particular class in “AND” mode. Here’s what that looks like:

#filters:has(#filter-or:checked):has(#aquatic:checked)  ~ #animals .aquatic,
#filters:has(#filter-or:checked):has(#bird:checked)     ~ #animals .bird,
...
#filters:has(#filter-or:checked):has(#reptile:checked)  ~ #animals .reptile {
  display: flex;
}

#filters:has(#filter-and:checked):has(#aquatic:checked) ~ #animals .animal:not(.aquatic),
#filters:has(#filter-and:checked):has(#bird:checked)    ~ #animals .animal:not(.bird),
...
#filters:has(#filter-and:checked):has(#reptile:checked) ~ #animals .animal:not(.reptile) {
  display: none;
}

It could probably enjoy an animation effect to make it clearer when items are added and removed², but that’s a consideration for another day.

Many developers would be tempted to use JavaScript to implement the client-side version of a filter like this. And in some cases, that might be the right option.

But it’s always worth remembering that:

• A CSS solution is almost-always more-performant than a JS one.
• A JS solution is usually less-resilient than a CSS one: a CDN failure, unsupported API, troublesome content-blocker or syntax error will typically have a much larger impact on JavaScript.
• For the absolutely maximum compatibility, consider what you can do in plain HTML, or on the server-side, and treat anything on the client-side as progressive enhancement.