Day 35: forgiving selectors

posted on November 11., 2022

It’s time to get me up to speed with modern CSS. There’s so much new in CSS that I know too little about. To change that I’ve started #100DaysOfMoreOrLessModernCSS. Why more or less modern CSS? Because some topics will be about cutting-edge features, while other stuff has been around for quite a while already, but I just have little to no experience with it.

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There's a difference between listing selectors in :where(), :is(), and :has() and listing them in a regular selector list.

Let's say you have a button with the class .button and you apply the following styles.

.button:hover {
  background-color: hwb(100 0% 20%);
}

<button class="button">I'm a button</hover>

Nothing special, the color just changes on hover. If you add more and different selectors to this rule, it still works.

.button:hover,
.button:focus,
#btn {
  background-color: hwb(100 0% 20%);
}

Here's were it gets interesting: If one of the selectors in your list of selectors is invalid, the whole rule becomes invalid and declarations apply to none of the selectors.

.button:hover,
.button:focus,
$btn {
  background-color: hwb(200 20% 0%);
}

Even using a pseudo-class that doesn't exist or that isn't supported by the browser invalidates the whole rule.

.button:hover,
.button:focus,
.button:touch {
  background-color: hwb(200 20% 0%);
}

So, a downside to using a selector list is that a single invalid or unsupported selector in the list of selectors invalidates the entire rule.

That's different when you're using :has(), :where() or :is() because they're so-called “forgiving selectors”. They just ignore the invalid selectors and apply the rules to the others.

button:where(:hover, :focus, $btn) {
  background-color: hwb(90 20% 20%);
}

button:where(:hover, :focus, :touch) {
  background-color: hwb(52 10% 20%);
}

This means that another benefit of using :is() or :where(), besides less lines of code and more control over specificity, is that you can use selectors that don't work in every browser in a list of selectors without having to worry that they invalidate the whole rule.

See on CodePen.