Two major features have made their way into modern CSS: container queries and the :has() selector. Together, they unlock new ways to write styles that are more flexible, modular, and context-aware.

What are Container Queries?

Container queries allow components to adapt based on the size of their container, not the entire viewport. This is a game-changer for building reusable, responsive components.

Here's a basic example:

.card {
  container-type: inline-size;
}

@container (min-width: 400px) {
  .card {
    flex-direction: row;
  }
}

With this, .card changes its layout only when its container is wider than 400px, not the whole screen. You can use it in grids, toolbars, cards, and nested components.

What is :has()?

The :has() selector is like a parent selector – it applies styles to elements based on their children.

Example:

form:has(input:invalid) {
  border: 2px solid red;
}

This applies a red border to the whole form if any of its inputs are invalid. Another use:

label:has(+ input:focus) {
  font-weight: bold;
}

This bolds a label when its associated input is focused.

Why This Matters

• Container queries help components behave independently, especially useful in design systems.
• :has() gives us a clean way to react to user interaction without JS.
• Support for both features is now solid in evergreen browsers.

Tools and Usage Tips

• Use @supports to check for availability:

  @supports (container-type: inline-size) {
    /* safe to use container queries */
  }
  

• Consider pairing with utility-first CSS like Tailwind (which now supports container queries in some setups).

Modern CSS keeps evolving fast. These features reduce the need for JavaScript or complex class toggles and make styling cleaner and more powerful.