I've known that :is and :where have existed for a while. But I hadn't seen a reason to change my autopilot of comma separation. Until recently.

I had a situation where I really wanted to use :has. But it's not quite ready for production code yet (it's not in Firefox at all, which is my favourite browser). The easy solution was to use JavaScript to add a class, which I could use instead of :has. But I thought I'd do some future-proofing and use :has anyway, so if someone has no JavaScript in their browser, but with a new enough browser where :has works, the styling I needed to do would work for them.

So that was fine and I ended up with this bit of CSS:

div:has(img),
div.has-image {
  /* styling goes here */
}

This worked fine in Chrome, but Firefox ignored all of that styling. That's because it didn't understand the first line, so it ignored the whole thing.

But :is works because if it doesn't understand something, it only ignores the bit it doesn't understand. So changing the code to this made it work in Firefox:

div:is(:has(img),has-image) {
  /* styling goes here */
}