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CSS Critical

Testing & Quality
css-critical

CSS Critical is an Agent Skill for testing & quality published in the thedaviddias/Front-End-Checklist repository on GitHub: a SKILL.md instruction file an AI agent can load to do this task.

Use when reviewing stylesheets, component styles, and responsive behavior related to Inline critical CSS for faster rendering. Check the rendered layout across breakpoints and interaction states before proposing a fix.

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QUICK START

How to use this skill

Bring this guide into your coding agent with a prompt tailored to the tool you use.

  1. Open your project in Codex.
  2. Copy and paste the prompt below.
  3. Review files and risks before approving any action.
Prompt to paste
Inspect the Agent Skill at https://github.com/thedaviddias/Front-End-Checklist/blob/HEAD/skills/css-critical/SKILL.md and all supporting files. Report provenance, permissions, scripts, dependencies, and security risks. Propose an exact installation into .agents/skills/css-critical, but wait for my approval before writing files or running scripts.

Copying this prompt does not install or run the skill. Review third-party files before use.

Inline critical CSS for faster rendering

Critical CSS eliminates render-blocking resources, enabling browsers to paint content immediately—directly improving LCP and FCP scores.

Quick Reference

  • Inline ~14KB of critical above-the-fold CSS in
  • Use tools: critical, critters, or Lighthouse to extract
  • Load remaining CSS asynchronously after page load
  • Automate in build process—don't manually maintain

Check

Analyze this page's CSS loading strategy to identify critical above-the-fold styles that should be inlined and non-critical styles that can be loaded asynchronously.

Fix

Extract and inline critical CSS for above-the-fold content, then load remaining CSS asynchronously to improve initial page render performance.

Explain

Explain how critical CSS improves Core Web Vitals by reducing render-blocking resources and enabling faster First Contentful Paint and Largest Contentful Paint.

Code Review

Review stylesheets, component styles, and responsive states related to Inline critical CSS for faster rendering. Flag exact selectors, declarations, or breakpoints that violate the rule in the rendered UI.

For full implementation details, code examples, and framework-specific guidance, see references/rule.md.

Rule page: https://frontendchecklist.io/en/rules/css/css-critical