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core-css-z-index

CSS Z-index

Last reviewed May 28, 2026 Content v20260528
Track mode
iframe_html
Means
HTML preview sandbox
Reading
~1 min
Level
advanced

This lesson

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

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

You will apply CSS Z-index 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.

z-index controls stacking order within stacking contexts. Bigger number is not global by default.

Stacking context triggers

  • Positioned elements with z-index.
  • Transforms, opacity < 1, filters, and some other properties.
  • Root and isolation boundaries.

Pitfall

A child with huge z-index cannot escape parent stacking context if parent is below another context.

Important interview questions and answers

  1. Q: Why can z-index: 9999 still be hidden?
    A: It may be trapped inside a lower stacking context.
  2. Q: How to debug layering bugs?
    A: Inspect stacking contexts in DevTools and simplify layering hierarchy.
  3. Q: Team strategy for overlays?
    A: Define a z-index scale/token system for modal, dropdown, tooltip tiers.

Pitfall: Check cascade order—author stylesheet loses to inline styles and !important surprises.

Interview tip Lesson completion confidence

Can you explain this lesson in 30 seconds without reading notes?

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Playground

Runs in your browser in a sandboxed frame. Backend runners appear when this track’s profile allows them.

Check yourself

Multiple choice — immediate feedback.

Discussion

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Starter discussion topics

  • What part of this lesson needs a second read?
  • What would you try differently in a real project?

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