CSS At-rules

Last reviewed May 28, 2026 Content v20260528

This lesson

This lesson teaches CSS At-rules—the ideas, syntax, and habits you need before moving on in CSS.

Without a solid grasp of CSS At-rules, you will repeat mistakes in CSS exercises and on real pages or scripts.

You will apply CSS At-rules in contexts like: All browser UIs, component libraries, marketing sites, and many native apps that reuse web views.

Read the lesson, edit HTML/CSS in the playground, press Run to preview, then answer the lesson MCQs.

When intermediate lessons feel comfortable and you are ready for production-style trade-offs.

At-rules control stylesheet structure and conditional behavior such as media queries, keyframes, supports, and font-face.

Q: Why avoid heavy @import chains?
A: Can complicate loading performance and dependency tracing.
Q: @supports role?
A: Progressive enhancement based on feature capability.
Q: @keyframes use case?
A: Defining multi-step animation states.

Practice: Change one property in the playground and observe cascade + layout in DevTools.

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