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 skills add ccheney/robust-skills --skill modern-cssgit clone --depth 1 https://github.com/ccheney/robust-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/ccheney/robust-skills/modern-css)<a href="https://agentmods.dev/skills/ccheney/robust-skills/modern-css"><img src="https://agentmods.dev/badge/skills/ccheney/robust-skills/modern-css/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/ccheney/robust-skills/modern-css"><img src="https://agentmods.dev/badge/skills/ccheney/robust-skills/modern-css.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
What 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 5.1 | $0.00038 | $0.00690 |
| Opus 5.5 | $0.00015 | $0.00276 |
| Sonnet 5.5 | $0.00008 | $0.00138 |
| Haiku 4.5 | $0.00004 | $0.00069 |
Grade A, and why
modern-css 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 1mo 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.
How it starts
The opening of the file, as written. The whole thing — 51 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Modern CSS
Choose native CSS features that fit the requested behavior and supported browsers. The user's explicit instructions take precedence over this skill's guidelines. Preserve the project's framework, preprocessor, tokens, and styling conventions unless changing them is part of the request.
Resolve the styling problem
Use the affected markup/styles, desired appearance or interaction, and browser targets as input. Infer targets from project configuration when available. If missing support information changes the implementation, ask for it or state a conservative fallback.
Bundled support tables are dated research snapshots, not a current compatibility guarantee. Check MDN compatibility data or Web Platform Status for the particular feature when support matters.
| Need | Starting point |
|---|---|
| Rows and columns, or aligned nested tracks | Grid / Subgrid |
| One-dimensional arrangement | Flexbox |
| Layout based on a component's available space | Container queries |
| Cascading overrides or specificity conflict | Layers, scope, selector specificity |
| Themes and derived colors | Custom properties, color functions, color-scheme |
| Motion, popovers, or scroll effects | Feature-specific reference and fallback |
Keep the result usable
- Preserve layout and interaction in supported browsers. Use progressive enhancement or feature detection where a feature is unavailable.
- Match cascade changes to the existing origin, importance, and layer ordering; adding a layer can change precedence.
- Respect reduced-motion preferences and preserve focus/keyboard behavior when changing interactive components.
- Use modern CSS as an option, not a mandate to remove JavaScript or rebuild working controls. Support and equivalent behavior determine whether a replacement fits.
- Scope resets and stylesheet reorganization to the task; a component edit does not require a global reset.
Deliver the requested style change or explanation and verify affected layout/states when implementing. State unverified browser limitations rather than treating an example as proof of support.
What ships with it
10 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 1mo ago Changed · -124 lines · -143 tokens per session f59b75ad6607
- 1mo ago First seen · 175 lines · 181 tokens per session scan A 3b4b5f3fe60f
modern-css is a skill published in the GitHub repository ccheney/robust-skills (61 stars, last pushed 21d ago), licensed MIT. It adds 38 tokens to every session and 690 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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