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There is a lot of hype on the :is() pseudo-selector lately, probably because now that Safari 14 has it, it’s supported across all the major browsers. You’ve got Miriam tweeting about it, Kevin Powell doing a video, Šime getting it into the Web Platform News, and Robin mentioning it. Bramus really puts a point on it with these “three important facts”:

1. The selector list of :is() is forgiving
2. The specificity of :is() is that of its most specific argument
3. :is() does not work with pseudo-element selectors (for now)

Plus, of course, it’s main functionality which is making otherwise rather verbose, complex, and error-prone selectors easier to write. The specificity thing is extra-interesting. Miriam notes some trickery you could do with it, like juicing up specificity without actually selecting anything.

Say you wanted to use the .button class to select, but give it a ton of specificity

:is(.button, #increase#specificity) {
  /* specificity is now (0, 1, 0, 0) instead of (0, 0, 1, 0)
}

I’ve done silly stuff like this in the past:

.button.button.button {
  /* forcing the selector to be (0, 0, 3, 0) instead of (0, 0, 1, 0) */
  /* doesn't actually require element to have three button classes lol */
}

The :is() trick seems a little more understandable to me.

But what if you want to go the other way with specificity and lower it instead? Well, that’s the whole point of the :where() pseudo-selector. Functionally, it’s exactly the same as :is(). You give it a comma-separated list of things to select as part of the selector chain, and it does, with the same forgiving nature. Except, the specificity of the entire :where() part is zero (0).

Kevin showed off an interesting gotcha with :is() in the video:

.card :is(.title, p) {
  color: red;
}

.card p {
  color: yellow;
}

You might think yellow will win out here, but the presence of .title in that :is() selector on top makes the specificity of that selector (0, 0, 2, 0) winning out over the (0, 0, 1, 1) below.

This is where we could consider using :where() instead! If we were to use the :where() pseudo-selector instead:

.card :where(.title, p) {
  color: red;
}

.card p {
  color: yellow;
}

Then yellow would win, because the top selector lowers to (0, 0, 1, 0) specificity, losing to the bottom selector’s (0, 0, 1, 1).

Which one should you use? Ya know what, I’m not really sure if there is super solid time-tested advice here. At least we have both options available, meaning if you get into a pickle, you’ve got tools to use. Time has taught me that keeping specificity low is generally a healthier place to be. That gives you room to override, where if you’re riding high with specificity you have fewer options. But the zero specificity of :where() is pretty extreme and I could see that leading to confusing moments, too. So my gut is telling me you might wanna start with :is(), unless you notice you need to mix in a higher-specificity selector; if you do, back off to :where().

The post :where() has a cool specificity trick, too. appeared first on CSS-Tricks.

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