Is vs Where in CSS Specificity: Key Differences and Usage

The CSS :is() pseudo-class matches any selector inside it and contributes to specificity based on its most specific argument, while :where() also matches any selector inside but always has zero specificity. Use :where() to avoid increasing specificity and :is() when you want to keep specificity from the matched selectors.

Quick Comparison

Here is a quick side-by-side comparison of :is() and :where() in CSS specificity and usage.

Feature	:is()	:where()
Purpose	Matches selectors, specificity from most specific argument	Matches selectors, always zero specificity
Specificity Impact	Takes highest specificity of arguments	Always zero specificity regardless of arguments
Use Case	When you want to group selectors and keep specificity	When you want to group selectors without increasing specificity
Browser Support	Modern browsers (Chrome, Firefox, Safari, Edge)	Modern browsers (Chrome, Firefox, Safari, Edge)
Example	:is(.btn, .link) { color: red; }	:where(.btn, .link) { color: red; }

Key Differences

The main difference between :is() and :where() lies in how they affect CSS specificity. :is() calculates specificity based on the most specific selector inside its parentheses. For example, if one selector inside :is() has a class and another has an element, the class selector's specificity is used.

On the other hand, :where() always has zero specificity no matter what selectors it contains. This means it never increases the specificity of the rule it is part of. This is useful when you want to apply styles broadly without overriding more specific rules unintentionally.

Both pseudo-classes simplify writing selectors by grouping multiple selectors, but choosing between them depends on whether you want to keep or reset specificity. :is() keeps specificity, so it can override less specific rules, while :where() avoids increasing specificity, making it easier to override later.

Code Comparison

Here is an example showing how :is() works with specificity:

css

/* Using :is() with class and element selectors */
:is(.btn, button) {
  color: blue;
}

/* This rule has specificity of a class selector because .btn is more specific than button */

Output

Text inside elements with class 'btn' or <button> elements will be blue.

:where() Equivalent

The same example using :where() will not increase specificity:

css

/* Using :where() with class and element selectors */
:where(.btn, button) {
  color: green;
}

/* This rule has zero specificity regardless of selectors inside :where() */

Output

Text inside elements with class 'btn' or <button> elements will be green, but this style can be easily overridden.

When to Use Which

Choose :is() when you want to group selectors and keep the highest specificity among them, so your styles can override less specific rules reliably.

Choose :where() when you want to group selectors but keep specificity zero, making your styles easy to override and avoiding unintended specificity conflicts.

In short, use :is() for stronger, grouped selectors and :where() for flexible, low-specificity grouping.

Key Takeaways

:is() uses the highest specificity of its selectors, affecting CSS specificity.

:where() always has zero specificity, never increasing specificity.

Use :is() to group selectors when you want to keep or increase specificity.

Use :where() to group selectors without increasing specificity for easier overrides.

Both simplify selector writing but serve different specificity needs.