Getting it into your agent
There is no command for this one: it runs only inside a plugin, and the catalogue could not identify which plugin ships it. The source is linked below.
Wrote 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/dublyo/saas-skills/mdn-web-best-practices)<a href="https://agentmods.dev/skills/dublyo/saas-skills/mdn-web-best-practices"><img src="https://agentmods.dev/badge/skills/dublyo/saas-skills/mdn-web-best-practices/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/dublyo/saas-skills/mdn-web-best-practices"><img src="https://agentmods.dev/badge/skills/dublyo/saas-skills/mdn-web-best-practices.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 | $0.00086 | $0.01335 |
| Opus 5 | $0.00043 | $0.00668 |
| Sonnet 5 | $0.00017 | $0.00267 |
| Haiku 4.5 | $0.00009 | $0.00134 |
Grade A, and why
mdn-web-best-practices 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 12d 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 — 121 lines — stays where its author put it; the contents beside it link to each section on GitHub.
MDN Web Best Practices
Operating Model
Use a spec-first workflow for web work unless the user already provided a precise implementation request. Default to:
- Clarify the target user, page/app type, core flow, framework, and validation bar.
- Inspect the current repo before choosing patterns or adding dependencies.
- Route to the smallest relevant reference files below.
- Use the local MDN checkout as the standards source of truth.
- Design context-aware UI, then implement against MDN-backed web platform rules.
- Validate build/tests, browser behavior, responsiveness, accessibility, console, performance, and security before finishing substantial work.
For audit-only requests, skip implementation unless the user asks for fixes. Lead with findings, file/line references, risk, and evidence.
Source Of Truth
Primary MDN source:
/Users/dribrahimm/skills/mozilla/content/files/en-us/web
Fallback source:
https://github.com/mdn/content/tree/main/files/en-us/web
Use the local source first. Use the GitHub fallback only when the local path is missing or the needed MDN topic is not present locally. When browser behavior, API support, accessibility semantics, or security requirements are uncertain, search MDN instead of relying on memory.
Reference Routing
Read only the references needed for the task:
references/mdn-routing.md- local MDN search rules, topic map, and fallback behavior.references/html.md- semantic HTML, document structure, media, interactive elements.references/css.md- layout, responsive CSS, media queries, container queries, motion.references/javascript.md- DOM, events, fetch, storage, browser APIs, progressive enhancement.references/accessibility.md- keyboard, focus, labels, ARIA restraint, reduced motion.references/forms.md- native forms, validation, errors, uploads, autofill, mobile inputs.references/performance.md- rendering, loading, images, fonts, animations, Core Web Vitals.references/security.md- XSS, CSP, cookies, CORS, storage, permissions, embeds.references/design-validation.md- design quality bar and final QA expectations.
What ships with it
12 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.
- agents/openai.yaml 285 B
- references/accessibility.md 1.4 KB
- references/css.md 1.8 KB
- references/design-validation.md 1.7 KB
- references/forms.md 1.6 KB
- references/html.md 1.7 KB
- references/javascript.md 1.6 KB
- references/mdn-routing.md 2.5 KB
- references/performance.md 1.4 KB
- references/security.md 1.7 KB
- scripts/search_mdn.py 3.4 KB runs code
- scripts/web_audit_checklist.py 6.7 KB runs code
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.
- 12d ago First seen · 121 lines · 86 tokens per session scan A 150025ed1fe7
mdn-web-best-practices is a skill published in the GitHub repository dublyo/saas-skills (10 stars, last pushed 2mo ago), licensed MIT. It adds 86 tokens to every session and 1,335 once invoked, about $0.0004 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 2026-08-31.
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