Skills

Skill 6 of 6

Web Quality Audit

Run an evidence-led web quality audit covering performance, accessibility, SEO, best practices, and agentic browsing.

5 minutes · 1,101 words · 15 sections

Install

$ npx skills add addyosmani/web-quality-skills --skill web-quality-audit

$ npx skills add addyosmani/web-quality-skills

» /plugin marketplace add addyosmani/web-quality-skills

The first command installs just this skill, by the name in its SKILL.md; the second installs the whole repository.

Comprehensive quality review that combines live browser evidence with source inspection. Covers Performance, Accessibility, SEO, Best Practices, and Agentic Browsing without treating an aggregate score as proof of quality.

Lighthouse 13+. The Performance category now uses shared Performance Insights across Lighthouse and the DevTools Performance panel (announcement (opens in a new tab)). Follow current insight names and evidence. Do not require removed audit IDs or automatically recreate their recommendations; some were retired because they were noisy, inactionable, or easy to over-recommend.

How it works

Establish the audit target: representative URLs, important states and journeys, public versus authenticated access, and mobile/desktop scope.
If a page can run, read the measurement workflow (opens in a new tab) and collect a minimal live baseline before searching the codebase broadly.
Use runtime failures to localize source inspection. Keep measured findings separate from hypotheses found only in code.
Categorize by user impact and confidence, then make or recommend specific fixes.
Re-run equivalent automated checks and the affected manual flows. Report what is verified and what still needs field or human validation.

Tool routing

Use the best capability already available; do not block the audit on optional setup.

Need	Preferred route	Fallback
Performance and Core Web Vitals	Record a browser performance trace and analyze focused insights; with Chrome DevTools MCP, use `performance_start_trace` then `performance_analyze_insight`	Lighthouse CLI or PageSpeed Insights lab data
Real-user performance	CrUX values included in current DevTools trace summaries	PageSpeed Insights/CrUX Vis; direct CrUX API only when a key is already available or automation is requested
Accessibility, SEO, Best Practices, Agentic Browsing	Run a live Lighthouse audit; with Chrome DevTools MCP, use `lighthouse_audit`	Category-specific Lighthouse CLI audits plus manual checks
Rendered semantics and interaction	Inspect the accessibility tree and exercise the UI; with Chrome DevTools MCP, use `take_snapshot` and focused `evaluate_script`	Browser/manual testing
Source smoke test	`scripts/analyze.sh <path>`	Direct source inspection

Chrome DevTools MCP’s lighthouse_audit intentionally excludes performance. Its navigation mode reloads the page; use snapshot mode when preserving the current authenticated or user-created state matters. The static analyzer is a fast smoke test, not a substitute for a rendered-page audit.

Audit categories

Performance

Core Web Vitals — Must pass for good page experience:

LCP (Largest Contentful Paint) < 2.5s. The largest visible element must render quickly. Optimize images, fonts, and server response time.
INP (Interaction to Next Paint) < 200ms. User interactions must feel instant. Reduce JavaScript execution time and break up long tasks.
CLS (Cumulative Layout Shift) < 0.1. Content must not jump around. Set explicit dimensions on images, embeds, and ads.

Resource Optimization:

Compress images. Use WebP/AVIF with fallbacks. Serve correctly sized images via srcset.
Minimize JavaScript. Remove unused code. Use code splitting. Defer non-critical scripts.
Optimize CSS. Extract critical CSS. Remove unused styles. Avoid @import.
Efficient fonts. Use font-display: swap. Preload critical fonts. Subset to needed characters.

Loading Strategy:

Preconnect to origins. Add <link rel="preconnect"> for third-party domains.
Preload critical assets. LCP images, fonts, and above-fold CSS.
Lazy load below-fold content. Images, iframes, and heavy components.
Cache effectively. Long cache TTLs for static assets. Immutable caching for hashed files.

Accessibility

Perceivable:

Text alternatives. Every <img> has meaningful alt text. Decorative images use alt="".
Color contrast. Minimum 4.5:1 for normal text, 3:1 for large text (WCAG AA).
Don’t rely on color alone. Use icons, patterns, or text alongside color indicators.
Captions and transcripts. Video has captions. Audio has transcripts.

Operable:

Keyboard accessible. All functionality available via keyboard. No keyboard traps.
Focus visible. Clear focus indicators on all interactive elements.
Skip links. Provide “Skip to main content” for keyboard users.
Sufficient time. Users can extend time limits. No auto-advancing content without controls.

Understandable:

Page language. Set lang attribute on <html>.
Consistent navigation. Same navigation structure across pages.
Error identification. Form errors clearly described and associated with fields.
Labels and instructions. All form inputs have associated labels.

Robust:

Valid HTML. No duplicate IDs. Properly nested elements.
ARIA used correctly. Prefer native elements. ARIA roles match behavior.
Name, role, value. Interactive elements have accessible names and correct roles.

SEO

Crawlability:

Valid robots.txt. Doesn’t block important resources.
XML sitemap. Lists all important pages. Submitted to Search Console.
Canonical URLs. Prevent duplicate content issues.
No noindex on important pages. Check meta robots and headers.

On-Page SEO:

Unique title tags. Make each title descriptive and concise; display truncation varies by device and result type.
Meta descriptions. Write useful, page-specific summaries; search engines may choose a different snippet.
Heading hierarchy. The primary heading is descriptive and the structure is logical; do not fail valid HTML solely for using more than one <h1>.
Descriptive link text. Not “click here” or “read more”.

Technical SEO:

Mobile-friendly. Responsive design. Tap targets ≥ 48px.
HTTPS. Secure connection required.
Page experience signals. Use field Core Web Vitals as evidence, without promising a ranking change.
Structured data. JSON-LD for rich snippets (Article, Product, FAQ, etc.).

Best practices

Security:

HTTPS everywhere. No mixed content. HSTS enabled.
No vulnerable libraries. Keep dependencies updated.
CSP headers. Content Security Policy to prevent XSS.
No exposed source maps. In production builds.

Modern Standards:

No deprecated APIs. Replace document.write, synchronous XHR, etc.
Valid doctype. Use <!DOCTYPE html>.
Charset declared. <meta charset="UTF-8"> as first element in <head>.
No browser errors. Clean console. No CORS issues.

UX Patterns:

No intrusive interstitials. Especially on mobile.
Clear permission requests. Only ask when needed, with context.
No misleading buttons. Buttons do what they say.

Agentic browsing

Use the Lighthouse Agentic Browsing results as technical signals for how well assistants can understand and interact with the rendered page.

Accessible interaction surface. Semantic HTML, labels, names, roles, and states must expose meaningful controls in the accessibility tree.
WebMCP integrations are valid when present. Review registered tools, schemas, and form coverage; do not add WebMCP solely to raise an audit score.
llms.txt is optional. A valid file may help compatible tools discover curated content, but a Lighthouse pass does not prove that search or AI products will ingest, rank, or cite it.
Keep this category separate from SEO claims. Agentic browsability is not evidence of search ranking or AI visibility.

Severity levels

Level	Description	Action
Critical	Security vulnerabilities, complete failures	Fix immediately
High	Core Web Vitals failures, major a11y barriers	Fix before launch
Medium	Performance opportunities, SEO improvements	Fix within sprint
Low	Minor optimizations, code quality	Fix when convenient

Audit output format

When performing an audit, structure findings as:

## Audit results

### Evidence
| Signal | Scope/conditions | Result | Source |
|--------|------------------|--------|--------|
| LCP | URL, phone, p75/28 days | 3.1s (needs improvement) | CrUX |
| Accessibility | URL, mobile navigation | 92 | Lighthouse |

### Critical issues (X found)
- **[Category]** Issue description. File: `path/to/file.js:123`
  - **Impact:** Why this matters
  - **Evidence:** Measured failure, runtime observation, or source hypothesis
  - **Fix:** Specific code change or recommendation

### High priority (X found)
...

### Summary
- Performance: measured status and X findings
- Accessibility: automated status, X findings, manual checks pending/passed
- SEO: X findings
- Best Practices: X findings
- Agentic Browsing: X findings or not available

### Recommended priority
1. First fix this because...
2. Then address...
3. Finally optimize...

### Verification
- Re-run results under the same conditions
- Manual checks completed
- Field validation still pending

Quick checklist

Before every deploy

Not done: Core Web Vitals passing
Not done: No accessibility errors (axe/Lighthouse)
Not done: No console errors
Not done: HTTPS working
Not done: Meta tags present

Weekly review

Not done: Check Search Console for issues
Not done: Review Core Web Vitals trends
Not done: Update dependencies
Not done: Test with screen reader

Monthly deep dive

Not done: Full Lighthouse audit
Not done: Performance profiling
Not done: Accessibility audit with real users
Not done: SEO keyword review

References

For detailed guidelines on specific areas:

About this skill

Trigger

Run an evidence-led web quality audit covering performance, accessibility, SEO, best practices, and agentic browsing. Use when asked to "audit my site", "review web quality", "run lighthouse audit", "check page quality", or "optimize my website".

The verbatim description from this skill’s front matter — the string an agent matches on to decide whether to load it.

Licence: MIT
Source: skills/web-quality-audit/SKILL.md

Colophon for this issue

Colophon

How this issue was made

Source

github.com/addyosmani/web-quality-skills, branch main, last pushed 24 August 2026.

Licence

MIT — the text of every skill is reproduced unmodified, frontmatter included, under the upstream licence.

Discovery

6 skills found by walking the repository tree for SKILL.md, not by matching a directory convention. One layout observed: skills/*/SKILL.md.

Issue colours

Resolved from a deterministic hash of the owner name. Two accent tones are generated per issue and each is proven against its own ground before it ships: a single accent that passes AA on both light and dark paper is arithmetically impossible.

Heading repairs

1 repair applied to this skill so the document has one h1 and no skipped levels:

Removed “Web quality audit”, a leading h1 that duplicated the skill title.

Spec compliance

Every skill satisfies the Agent Skills specification’s frontmatter rules.

Images

Images inside a skill come from the upstream repository. Where the author gave no alternative text we mark the image decorative rather than inventing a description — a plausible caption we made up is worse than none for the reader who depends on it.

Marketplace

A plugin manifest is published at .claude-plugin/marketplace.json by Addy Osmani, declaring 1 plugin. It is read for editorial metadata only — never as the skill index, which is always the repository tree.

Signal

Install counts come from skills.sh. They measure downloads, not quality, and an unranked repository is not an unread one.

Agent surfaces

The whole issue is available as one markdown document at /addyosmani/web-quality-skills.md, and each skill at its own .md URL.

Publication

Set by Skills Docs from the source repository. Body text is Literata at the reader’s chosen size and measure; code is Geist Mono. Nothing on this page was written by us except this paragraph.