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The CSS :where() Function

This article demonstrates how to use the CSS :where() function within CSS style sheets. Included are code, definitions, and examples.

Contents

What is CSS :where()?

The :where() function is a functional pseudo-class that takes a list of selectors as its argument and applies the styles to the selected HTML elements in the list.

Syntax

:where( <complex-selector-list> )

The :where() function parameter is a <complex-selector-list> data type and should be replaced with an example value such as: :where(:not(:hover)).

Examples

The examples below use selectors and selector list values for the parameter of the :where() function.

:where(:not(:hover))
:where(ol, ul, menu:unsupported)
:where(section, aside, footer) a
:where(header, main, footer) a:hover
:where(header, footer) > p
:where(button, a)
:where(.light-theme, .bright-theme)
:where(ol[class])
:where(:valid, :unsupported)
:where(#section) h2

Usage

You can apply the :where() function to any element by using CSS selectors and adding styles.

:where(ol, ul) :where(ol, ul) ol {
    list-style-type: lower-greek;
    color: green;
}

:where(section) a:hover {
  color: blue;
  text-decoration: underline;
}

article :where(footer, aside) > p {
  color: orange;
}

:where(button, a) {
  color: purple;
}

main :where(h1, h2, h3) {
  color: orange;
}

Use Cases of :where()

  • Overriding Styles:where() is helpful for creating filters in a selector while keeping associate selectors easy to override.
  • Shorting Selector Lists:where() can shorten CSS style selectors by reducing redundancy and is very useful in shortening CSS code.
header a:hover,
main a:hover,
aside:hover,
footer a:hover {
  color: blue;
  text-decoration: underline;
}

The above CSS selectors can be reduced by using the :where() function because the selectors can be turned into a list. The example below reduced the characters from 100 to 89, an 11% reduction.

:where(header, main, aside, footer) a:hover {
  color: blue;
  text-decoration: underline;
}

Using the :where() function gives us the advantage of creating shorter and more readily understandable lists.

Specificity

CSS :where() function is similar to :is() syntax and functionality. However, the difference between :is() and :where() is that :where() is their specificity.

The CSS :where() function always has a specificity of zero. This means that the :where() pseudo-class or its argument list does not contribute to the specificity of the selector.

An example of specificity failing.

a:not(:hover) {
  text-decoration: none;
  color: blue;
}

nav a {
  /* Has no effect */
  text-decoration: underline;
  color: orange;
}

However, by using :where() you can explicitly declare selector intent:

a:where(:not(:hover)) {
  text-decoration: none;

  color: blue;
}

nav a {
  /* Works now! */
  text-decoration: underline;

  color: orange;
}

Forgiving Selectors Lists

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The :where() function is forgiving in that each selector within the comma-separated list :where(x, y, z) is ignored if it is an invalid selector. With :where() being forgiving, this means invalid selectors within the pseudo-class don’t disable the entire selector like they normally do.

Syntactically the comma list is a <forgiving-selector-list> data value which is equivalent to <any-value>? The browser then parses the forgiving selector list into valid selectors to obtain its actual value.

Below, the totally:fake selector is ignored while the heading selector isn’t.

:where(<forgiving-selector-list> | <any-value>?) {
  /* Some element */
}

/* Invalid "totally:fake is ignored */
:where(heading, totally:fake) {
  /* will still select heading */
}

Refactoring Using :where()

When using :where(), we can place it at the beginning, middle, or end of a selector. This allows us to use the function to combine selectors in different ways.

Here is an example of multiple styles and selectors.

/* first list */
header a:hover,
section: a:hover,
main a:hover,
footer a:hover {
  color: green;
  text-decoration: underline;
}

/* second list */
article header > p,
article section > p,
article footer > p {
  color: blue;
}

/* third list */
.light-theme button,
.light-theme a {
  color: black;
}

In the first CSS style block, we set the green and underline styles to be applied to the header, section, main, and footer elements whenever the link element is hovered.

In the second list, we specify that the article’s header, section, and footer paragraph elements should be styled with blue text. 

The last list sets the .light-theme’s link and button elements to have black color text.

Now we can use the :where() function to reduce the CSS.

/* first list */
/* at the beginning */
:where(header, section, main, footer) a:hover {
  color: green;
  text-decoration: underline;
}

/* second list */
/* in the middle */
article :where(header, section, footer) > p {
  color: blue;
}

/* third list */
/* at the end */
.light-theme :where(button, a) {
  color: black;
}

We replaced each selector group with a :where() function. This resulted in fewer selectors, as the selector’s redundant parts were removed.

Using :where() To Target Elements

Using another example, we can use the :where() function to target different links within HTML elements such as the header, sections, and footers.

Targeting Link Elements With CSS where()
Targeting Link Elements With CSS where()

HTML

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

  <section class="where-styling">
    <p>
      Here is my section content. This
      <a href="https://appcode.app">also contains a link</a>.
    </p>
  </section>

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

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

CSS

html {
  font-family: sans-serif;
  font-size: 150%;
  text-align: center;
  height: 100%;
  margin: 0;
  -webkit-box-align: center !important;
  -ms-flex-align: center !important;
  align-items: center !important;
  -webkit-box-pack: center !important;
  -ms-flex-pack: center !important;
  justify-content: center !important;
  display: -webkit-box !important;
  display: -ms-flexbox !important;
  display: flex !important;
}

:where(header.where-styling) a {
  color: blue;
}

:where(section.where-styling) a {
  color: green;
}

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

The last CSS selector uses a :where() to override the last link element color from its default orange.

/* Overwriting the last link element in the footer */
:where(footer.where-styling) a {
  color: red;
}

Try It

Below, we show how to use the :where() function to style and target different levels of unordered and ordered lists by supplying selectors in the parameter :where().

Using The CSS where() Function
Using The CSS where() Function

HTML

<ol>
  <li>Shapes</li>
  <ul>
    <li>Square</li>
    <li>
      <ul>
        <li>Color</li>
        <li>Size</li>
        <li>
          <ol>
            <li>X</li>
            <li>Y</li>
            <li>Z</li>
          </ol>
        </li>
        <li>Weight</li>
      </ul>
    </li>
    <li>Circle</li>
    <li>
      <ol>
        <li>Color</li>
        <li>Weight</li>
      </ol>
    </li>
  </ul>
</ol>

CSS

html,
body {
  height: 100%;
  margin: 0;
  -webkit-box-align: center !important;
  -ms-flex-align: center !important;
  align-items: center !important;
  -webkit-box-pack: center !important;
  -ms-flex-pack: center !important;
  justify-content: center !important;
  display: -webkit-box !important;
  display: -ms-flexbox !important;
  display: flex !important;
  font-size: 18px;
}

ol {
  list-style-type: number;
  color: darkblue;
}

/* Not applied to ol, because of lower specificity */
:where(ol, ul, menu:unsupported) :where(ol, ul) {
  color: green;
}

:where(ol, ul ul) :where(ol, ul ul) ol {
  list-style-type: lower-greek;
  color: brown;
}

:where(ol, ul ul) :where(ol, ul ul) {
  list-style-type: lower-alpha;
  color: #871f78;
}

Specifications

W3C CSS Selectors Level 4
#zero-matches

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