CSS Hooks
Source on GitHub


Is CSS Hooks widely supported in browsers?

The underlying mechanism of CSS Hooks leverages specific behaviors of CSS variables codified in the CSS Custom Properties Level 1 spec. As such, it is expected to work in any browser that supports custom properties:


Why can't hooks affect descendant elements?

This is primarily a limitation of inline styles, which are scoped strictly to the element to which they are applied (although individual properties can be inherited). Hooks don't add full CSS selector capabilities to inline styles. Instead, they allow you to toggle between arbitrary values depending on the boolean state of a CSS rule. The Introduction describes this mechanism and can help with understanding why the limitation exists.

Generally, the best solution is to apply inline styles directly to the descendant elements you want to affect, using hooks to change values based on surrounding context if necessary. For example, instead of using an (invalid) .group:hover * hook on the parent element to modify descendants' properties on hover, you would use a .group:hover & hook on each descendant element to change its properties based on the hover state of the .group ancestor.

Although we consider them edge cases, situations indeed exist where you can't control certain markup, and descendant selectors provide the only means for applying styles. For example, this is common with third-party libraries. For these cases, we simply recommend maintaining a global style sheet the "traditional" way. Using inline styles and hooks when possible can still help to keep this style sheet small enough to avoid maintainability issues.

Why don't hooks support pseudo-elements?

Most of the time, a selector or at-rule defines a condition under which a declaration block applies to an element. Hooks exploit the fact that these conditions can be encoded in CSS Variables for use within inline styles.

Pseudo-element selectors are different in nature. Rather than defining when a rule should apply to an existing DOM element, pseudo-element selectors create virtual elements, or target elements that don't correspond to markup.

Fortunately, in some cases pseudo-elements are unnecessary:

  • ::before and ::after are equivalent to physical elements added as the first and last children respectively.
  • Instead of ::first-letter, you can wrap the first letter in a <span> and apply inline styles to that.
  • You can use the :placeholder-shown pseudo-class (in a hook) to show a physical placeholder element conditionally, rather than styling ::placeholder.

When you must use a pseudo-element, you can simply maintain a small global style sheet the "traditional" way. Using inline styles and hooks for everything else will still help you to keep this small and maintainable. You could also consider using a custom property to make this approach more flexible and promote reuse.

What if my question isn't answered here?

Please start a discussion.