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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 overidden 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 Status Comment
Selectors Level 4
The definition of ':where()' in that specification.
Working Draft Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
:where()Chrome Full support 72
Disabled
Full support 72
Disabled
Disabled From version 72: this feature is behind the #enable-experimental-web-platform-features preference (needs to be set to Enabled). To change preferences in Chrome, visit chrome://flags.
Edge No support NoFirefox Full support 78
Full support 78
Full support 77
Notes Disabled
Notes Enabled by default in Firefox Nightly.
Disabled From version 77: this feature is behind the layout.css.is-where-selectors.enabled preference (needs to be set to enabled). To change preferences in Firefox, visit about:config.
IE No support NoOpera No support NoSafari Full support 14WebView Android No support NoChrome Android Full support 72
Disabled
Full support 72
Disabled
Disabled From version 72: this feature is behind the #enable-experimental-web-platform-features preference (needs to be set to Enabled). To change preferences in Chrome, visit chrome://flags.
Firefox Android Full support 79Opera Android No support NoSafari iOS Full support 14Samsung Internet Android No support No
Support for forgiving selector listChrome No support NoEdge No support NoFirefox Full support 82IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android Full support 82Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.

See also

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