:is() and :where() are used to simplify CSS selectors, the difference is that the former affects specificity, while the latter does not. :is() reduces duplicate code by combining multiple selectors, such as ::is(h1,h2,h3,p,li) a:hover matches any link that meets the criteria and adopts the highest internal specificity; while :where() also combines selectors but does not increase specificity, which is suitable for resetting styles or avoiding conflicts, such as ::where(.highlight,.feature) background color setting will not override other rules. Common uses of the two include theme switching, component style grouping and low specific global reset to improve code maintainability and scalability.

The :is() and :where() pseudo-classes in CSS selectors are used to group multiple selectors together, making your CSS more concise and easier to manage. They're especially useful when you want to apply the same styles to several different elements without repeating yourself.

What :is() Does and When to Use It

:is() lets you write shorter, cleaner selectors by grouping similar ones into one. It matches any element that fits at least one of the selectors inside its parentses.

For example, if you want to style links differently when they're inside headings, paragraphs, or lists, instead of writing:

 h1 a:hover,
h2 a:hover,
h3 a:hover,
pa:hover,
li a:hover {
  color: red;
}

You can simplify it to:

 :is(h1, h2, h3, p, li) a:hover {
  color: red;
}

This makes your code easier to read and maintain. One thing to keep in mind: the specific of the entire selector is based on the most specific item inside the :is() . So if you have something like #main :is(.class, div) , the specificity will be high because of the ID selector.

How :where() Is Different

:where() works just like :is() in terms of matching elements — it also matches any element that fits one of the selectors inside. But there's one big difference: it doesn't affect specific .

This can be really handy when you want to write utility-like styles that shouldn't override other more specific rules unintentionally.

Take this example:

 :where(.highlight, .feature) {
  background-color: yellow;
}

No matter where .highlight or .feature appears in your HTML, this rule won't interfere with other more specific styles because its specific is zero.

So if you're building reset styles, theme layers, or design systems where avoiding conflicts matters, :where() can help keep things predictable.

Practical Uses for Both

Here are some real-world ways these pseudo-classes come in handy:

• ? Theme switching : You can use :is() to target multiple theme classes at once:

   :is(.theme-dark, .theme-blue) button {
    color: white;
  }

• ? Component-based styleling : Keep component styles grouped without repetition:

   :is(.card, .panel, .widget) {
    padding: 1rem;
    border-radius: 8px;
  }

• ?? Low-specific resets : Use :where() to safely reset styles across many elements:

   :where(*) {
    box-sizing: border-box;
    margin: 0;
    padding: 0;
  }

  These tricks help reduce bloat and make your CSS easier to scale.

  ---

  That's basically what :is() and :where() do — one helps you write smarter selectors with normal specification, the other does the same without affecting how your styles stack up. Not too flashy, but definitely useful once you start dealing with complex or large-scale stylesheets.

  The above is the detailed content of What is the purpose of the :is() and :where() pseudo-classes in CSS Selectors?. For more information, please follow other related articles on the PHP Chinese website!