:where()

The :where() CSS pseudo-class function takes a selector list as its argument, and selects any element that can be selected by one of the selectors in that list.

/* Selects any paragraph inside a header, main
   or footer element that is being hovered */
:where(header, main, footer) p:hover {
  color: red;
  cursor: pointer;
}

/* The above is equivalent to the following */
header p:hover,
main p:hover,
footer p:hover {
  color: red;
  cursor: pointer;
}

The difference between :where() and :is() is that :where() always has 0 specificity, whereas :is() takes on the specificity of the most specific selector in its arguments.

The specification defines :is() and :where() as accepting a forgiving selector list.

In CSS when using a selector list, if any of the selectors are invalid then the whole list is deemed invalid. When using :is() or :where() instead of the whole list of selectors being deemed invalid if one fails to parse, the incorrect or unsupported selector will be ignored and the others used.

:where(:valid, :unsupported) {
  ...
}

Will still parse correctly and match :valid even in browsers which don't support :unsupported, whereas:

:valid, :unsupported {
  ...
}

Will be ignored in browsers which don't support :unsupported even if they support :valid.

This example shows how :where() works, and also illustrates the difference between :where() and :is().

Take the following HTML:

<article>
  <h2>:is()-styled links</h2>
  <section class="is-styling">
    <p>Here is my main content. This <a href="https://mozilla.org">contains a link</a>.
  </section>

  <aside class="is-styling">
    <p>Here is my aside content. This <a href="https://developer.mozilla.org">also contains a link</a>.
  </aside>

  <footer class="is-styling">
    <p>This is my footer, also containing <a href="https://github.com/mdn">a link</a>.
  </footer>
</article>

<article>
  <h2>:where()-styled links</h2>
  <section class="where-styling">
    <p>Here is my main content. This <a href="https://mozilla.org">contains a link</a>.
  </section>

  <aside class="where-styling">
    <p>Here is my aside content. This <a href="https://developer.mozilla.org">also contains a link</a>.
  </aside>

  <footer class="where-styling">
    <p>This is my footer, also containing <a href="https://github.com/mdn">a link</a>.
  </footer>
</article>

In this somewhat-contrived example, we have two articles that each contain a section, an aside, and a footer. They differ by the classes used to mark the child elements.

To make selecting the links inside them simpler, but still distinct, we could use :is() or :where(), in the following manner:

html {
  font-family: sans-serif;
  font-size: 150%;
}

:is(section.is-styling, aside.is-styling, footer.is-styling) a {
  color: red;
}

:where(section.where-styling, aside.where-styling, footer.where-styling) a {
  color: orange;
}

However, what if we later want to override the color of links in the footers using a simple selector?

footer a {
  color: blue;
}

This won't work for the red links, because the selectors inside :is() count towards the specificity of the overall selector, and class selectors have a higher specificity than element selectors.

However, selectors inside :where() have specificity 0, so the orange footer link will be overridden by our simple selector.

:where( <complex-selector-list> )
where 
<complex-selector-list> = <complex-selector>#
where 
<complex-selector> = <compound-selector> [ <combinator>? <compound-selector> ]*
where 
<compound-selector> = [ <type-selector>? <subclass-selector>* [ <pseudo-element-selector> <pseudo-class-selector>* ]* ]!
<combinator> = '>' | '+' | '~' | [ '||' ]
where 
<type-selector> = <wq-name> | <ns-prefix>? '*'
<subclass-selector> = <id-selector> | <class-selector> | <attribute-selector> | <pseudo-class-selector>
<pseudo-element-selector> = ':' <pseudo-class-selector>
<pseudo-class-selector> = ':' <ident-token> | ':' <function-token> <any-value> ')'
where 
<wq-name> = <ns-prefix>? <ident-token>
<ns-prefix> = [ <ident-token> | '*' ]?  | 
<id-selector> = <hash-token>
<class-selector> = '.' <ident-token>
<attribute-selector> = '[' <wq-name> ']' | '[' <wq-name> <attr-matcher> [ <string-token> | <ident-token> ] <attr-modifier>? ']'
where 
<attr-matcher> = [ '~' |  |  | '^' | '$' | '*' ]? '='
<attr-modifier> = i | s

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