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; } 

: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.

Forgiving Selector Parsing

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.

Examples

Comparing :where() and :is()

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.

Note: You can also find this example on GitHub; see is-where.

Syntax

: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

Specifications

Specification
Selectors Level 4 # zero-matches

See also

:is() Selector list Web components

Last modified: Jan 17, 2022, by MDN contributors

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