Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/librefang/librefang-registry/css-expertnpx skills add librefang/librefang-registry --skill css-expertgit clone --depth 1 https://github.com/librefang/librefang-registryWhat it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5 | $0.00021 | $0.00648 |
| Opus 5 | $0.00010 | $0.00324 |
| Sonnet 5 | $0.00004 | $0.00130 |
| Haiku 4.5 | $0.00002 | $0.00065 |
Grade A, and why
css-expert scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
This is a copy
89% identical to css-expert — 3 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 43 lines — stays where its author put it; the contents beside it link to each section on GitHub.
CSS Expert
A front-end layout specialist with deep command of modern CSS, from flexbox and grid to container queries and cascade layers. This skill provides precise, standards-compliant guidance for building responsive, accessible, and maintainable user interfaces using the latest CSS specifications and best practices.
Key Principles
- Use flexbox for one-dimensional layouts (rows or columns) and CSS Grid for two-dimensional layouts (rows and columns simultaneously)
- Embrace custom properties (CSS variables) for theming, spacing scales, and any value that repeats or needs runtime adjustment
- Design mobile-first with min-width media queries, layering complexity as viewport size increases
- Prefer logical properties (inline-start, block-end) over physical ones (left, bottom) for internationalization-ready layouts
- Leverage the cascade intentionally with @layer declarations to control specificity without resorting to !important
Techniques
- Use flexbox justify-content and align-items for main-axis and cross-axis alignment; flex-wrap with gap for fluid card layouts
- Define CSS Grid layouts with grid-template-areas for named regions, and auto-fit/auto-fill with minmax() for responsive grids without media queries
- Create design tokens as custom properties on :root (--color-primary, --space-md) and override them in scoped selectors or media queries
- Use @container queries to style components based on their parent container size rather than the viewport
- Build animations with @keyframes and animation shorthand; prefer transform and opacity for GPU-accelerated, jank-free motion
- Apply transitions on interactive states (hover, focus-visible) with appropriate duration (150-300ms) and easing functions
- Use the :has() selector for parent-aware styling, :is()/:where() for grouping selectors with controlled specificity
Common Patterns
- Holy Grail Layout: CSS Grid with grid-template-rows (auto 1fr auto) and grid-template-columns (sidebar content sidebar) for header/footer/sidebar page structures
- Fluid Typography: clamp(1rem, 2.5vw, 2rem) for font sizes that scale smoothly between minimum and maximum values without breakpoints
- Aspect Ratio Boxes: Use the aspect-ratio property directly instead of the legacy padding-bottom hack for responsive media containers
- Dark Mode Toggle: Define color tokens as custom properties, swap them inside a prefers-color-scheme media query or a data-theme attribute selector
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 2d ago First seen · 43 lines · 21 tokens per session scan A e86b2abc97b8
css-expert is a skill published in the GitHub repository librefang/librefang-registry (11 stars, last pushed 8d ago), licensed MIT. It adds 21 tokens to every session and 648 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. It is 89% identical to css-expert, differing in 3 lines, and is treated as a copy.
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