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/ccheney/robust-skills/modern-cssnpx 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.svg" alt="Measured on agentmods" height="20"></a>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 | $0.00181 | $0.02774 |
| Opus 5 | $0.00090 | $0.01387 |
| Sonnet 5 | $0.00036 | $0.00555 |
| Haiku 4.5 | $0.00018 | $0.00277 |
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 5d 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 — 175 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Modern CSS
Pure native CSS for building interfaces — no preprocessors, no frameworks.
When to Use (and When NOT to)
Support statuses below are as of mid-2026. "Newly Baseline (date)" means all three engines ship it, but users on older browsers won't have it — keep graceful degradation. Verify anything critical on MDN or webstatus.dev.
| Use Freely (Baseline) | Feature-Detect / Progressive Enhancement |
|---|---|
| CSS Grid, Subgrid, Flexbox | Scroll-driven animations (Firefox: behind flag) |
| Container queries — size; style (newly Baseline May 2026) | Scroll-state queries (Chromium-only) |
:has(), :is(), :where() |
::scroll-button(), ::scroll-marker (Chromium-only) |
CSS Nesting, @layer |
Customizable <select> (Chromium; Safari 27 announced) |
@scope (newly Baseline Dec 2025) |
@function, if() (Chromium-only) |
@property (typed custom props) |
sibling-index(), sibling-count() (no Firefox) |
oklch(), color-mix(), light-dark(), relative color |
Typed attr() beyond content (Chromium-only) |
@starting-style, transition-behavior: allow-discrete |
Cross-document view transitions (no Firefox) |
| View transitions — same-document (newly Baseline Oct 2025) | interpolate-size, calc-size() (Chromium-only) |
| Anchor positioning (newly Baseline Jan 2026) | popover="hint", interestfor (Chromium-only) |
Popover API, <dialog>, invoker commands (newly Baseline Dec 2025) |
text-wrap: pretty, text-box (no Firefox) |
field-sizing: content (newly Baseline Jun 2026) |
reading-flow (Chromium-only) |
text-wrap: balance, linear() easing |
closedby on <dialog> (no Safari) |
Logical properties, ::details-content |
Grid Lanes / masonry (Safari 26.4 only; flags elsewhere) |
random() (Safari 26.2+ only), @mixin (no browser yet) |
CRITICAL: The Modern Cascade
Understanding how styles resolve is the single most important concept in CSS. The additions of @layer and @scope fundamentally changed the cascade algorithm.
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.
- 5d 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 (57 stars, last pushed 8d ago), licensed MIT. It adds 181 tokens to every session and 2,774 once invoked, about $0.0009 per session on Opus 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.
Other skills, from other repositories
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frontend-design
Guidance for distinctive, intentional visual design when building new UI or reshaping an existing one. Helps with aesthetic direction, typography, and making choices that don't read as templated defaults.
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Audit and fix animation performance issues including layout thrashing, compositor properties, scroll-linked motion, and blur effects. Use when animations stutter, transitions jank, or reviewing CSS/JS animation performance.
baseline-ui
Quickly deslop UI code by fixing spacing, hierarchy, typography, and small layout issues. Use when the interface needs a fast cleanup or polish pass.
frontend-design
创建具有高设计品质、可交付生产的前端界面。当用户要求构建 Web 组件、页面、海报或应用（如官网、落地页、仪表盘、React/Vue 组件、HTML/CSS 布局，或对任意 Web UI 进行样式/美化）时使用本技能。产出有创意、打磨到位且避免「AI 通用审美」的代码与界面设计。.
deploy
Convert per-page styled HTML prototypes (stardust under stardust/prototypes/, or claude-design / Mobirise / Relume / Lovable / v0 / Figma-derived pages, or JSX prototypes pre-rendered to HTML, often under samples/) into Edge Delivery Services (EDS / AEM) blocks and content pages, then deploy via DA. Each prototype…