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 s977043/river-review --skill modern-web-browser-compatgit clone --depth 1 https://github.com/s977043/river-reviewWrote 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/s977043/river-review/modern-web-browser-compat)<a href="https://agentmods.dev/skills/s977043/river-review/modern-web-browser-compat"><img src="https://agentmods.dev/badge/skills/s977043/river-review/modern-web-browser-compat/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/s977043/river-review/modern-web-browser-compat"><img src="https://agentmods.dev/badge/skills/s977043/river-review/modern-web-browser-compat.svg" alt="Reviewed on agentmods" width="80" 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 5.1 | $0.00045 | $0.01919 |
| Opus 5.5 | $0.00018 | $0.00768 |
| Sonnet 5.5 | $0.00009 | $0.00384 |
| Haiku 4.5 | $0.00005 | $0.00192 |
Grade A, and why
Modern Web Browser Compatibility + Baseline Awareness 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 — 118 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Goal / 目的
- 新しい Web API / CSS プロパティを差分で導入したときに、Baseline 状態 (Newly available / Widely available / Limited availability) とブラウザサポートを確認する起点を示す。
- progressive enhancement / feature detection / graceful degradation の有無を suggestion で問いかける。
- ブラウザサポート方針が repo に明文化されているなら、その方針との整合を優先する。
Non-goals / 扱わないこと
- 完全な caniuse データの代替（実際のサポート % は MDN / web-features を参照する起点として扱う）。
- polyfill 推奨そのもの（重い polyfill 追加は別レビュー / 別議論）。
- ブラウザサポート対象範囲のチーム合意形成（人間レビューへ）。
False-positive guards / 抑制条件
- repo に「Modern browsers only」「Evergreen browser support」等の明示があるなら、Widely available 級の API は指摘しない。
- transpiler / bundler (Babel / SWC) が target ブラウザを設定している場合、JS 構文系は対象外（CSS / Web API は別問題）。
- 既に
@supports/if ('xxx' in window)/try-catch等の feature detection が含まれている差分は指摘しない。 - progressive enhancement の fallback がコメント・docs で明示されている場合は指摘しない。
Rule / ルール
Baseline 状態の確認
- Newly available (緑、新規): 過去 30 ヶ月未満で全モダンブラウザに到達した機能。利用は OK だが、Safari 旧バージョン / IE 互換要件があれば fallback を確認。
- Widely available (青、安定): 30 ヶ月以上経過し全モダンブラウザで安定。基本的に suggestion なし。
- Limited availability (赤、未到達): 一部ブラウザで未実装 / experimental flag 越し。必ず fallback / feature detection を提案する。
JavaScript / Web API
Object.hasOwn/Array.prototype.findLast/structuredClone/Promise.anyなどの比較的新しい API は MDN / Baseline status の確認を促す。URL.parse(static) /Set.prototype.intersection/ iterator helper (map/filteron iterator) などの Limited availability 級は feature detection を提案する。- Service Worker / Web Workers / Push API のような環境依存 API は graceful degradation を促す。
CSS
:has()/@container/subgrid/text-wrap: balance/view-transition-nameなどの新 CSS は Baseline 確認を促す。color-mix()/oklch()/light-dark()/ Anchor Positioning などの 2024 級は@supportsfallback の検討を提案する。- Container Queries は Baseline Widely available 寄りだが、初期に書いたコードが古いブラウザ起動を含むかは確認する。
HTML
<dialog>の Web Platform Native 利用は Baseline Widely available（Safari 15.4+ / Chrome 37+ / Firefox 98+）→ 通常 suggestion なし。- Popover API (
popoverattribute) は Baseline Newly available（2024 〜）→ Limited availability エリアでの fallback を確認。 loading="lazy"/decoding="async"は Widely available。
What ships with it
8 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 First seen · 118 lines · 45 tokens per session scan A 9a90744781f3
Modern Web Browser Compatibility + Baseline Awareness is a skill published in the GitHub repository s977043/river-review (7 stars, last pushed today), licensed MIT. It adds 45 tokens to every session and 1,919 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 2026-09-03.
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