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/schalkneethling/webdev-agent-skills/semantic-htmlnpx skills add schalkneethling/webdev-agent-skills --skill semantic-htmlgit clone --depth 1 https://github.com/schalkneethling/webdev-agent-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/schalkneethling/webdev-agent-skills/semantic-html)<a href="https://agentmods.dev/skills/schalkneethling/webdev-agent-skills/semantic-html"><img src="https://agentmods.dev/badge/skills/schalkneethling/webdev-agent-skills/semantic-html.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 | $0.00051 | $0.05518 |
| Opus 5 | $0.00026 | $0.02759 |
| Sonnet 5 | $0.00010 | $0.01104 |
| Haiku 4.5 | $0.00005 | $0.00552 |
Grade A, and why
semantic-html 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 4d 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 — 604 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Semantic HTML
Write HTML that conveys meaning, serves all users, and respects the web platform.
When to Use This Skill
Use this skill when:
- Creating new components or page sections
- Reviewing markup for accessibility and semantics
- Deciding between native HTML elements and ARIA attributes
- Structuring documents with proper heading hierarchy
- Making interactive elements accessible
- Building forms with proper labelling and error handling
- Creating responsive tables
Core Principles
Content Realism
Design content is idealized. Real content is messy. Always account for:
- Long sentences and long words
- Images with varying aspect ratios and sizes
- Multi-language support (even if not planned—users can translate via browser)
- Dynamic content that changes in length and structure
Build components that handle real-world content gracefully, not just what looks good in design tools.
Landmarks-First Planning
Before diving into individual components, consider the full page structure. This allows you to:
- Identify key landmarks for assistive technology users
- Plan heading hierarchy across the document
- Make informed decisions about element choice
- Avoid overusing landmarks (which diminishes their usefulness)
Native Over ARIA
Follow the first rule of ARIA: if a native HTML element provides the semantics and behaviour you need, use it instead of adding ARIA to a generic element.
Red flag: High div count combined with high ARIA count on non-complex components signals reaching for patches rather than foundations.
Redundant ARIA
Adding ARIA to elements that already carry the correct semantics is noise—it clutters the code, can confuse assistive technology, and obscures genuine intent:
<!-- Redundant: <ul> already has list semantics -->
<ul role="list">
...
</ul>
<!-- Redundant: alt="" already suppresses the accessible name -->
<img src="avatar.png" alt="" role="presentation" />
<!-- Redundant: aria-label duplicates visible text the AT will already read -->
<span aria-label="Most Popular">Most Popular</span>
What ships with it
2 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.
- 4d ago First seen · 604 lines · 51 tokens per session scan A eee564149cdd
semantic-html is a skill published in the GitHub repository schalkneethling/webdev-agent-skills (7 stars, last pushed 4mo ago), licensed MIT. It adds 51 tokens to every session and 5,518 once invoked, about $0.0003 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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