Modern Web Guidance

A reference and search tool for modern web platform APIs. Use before implementing any HTML/CSS/JS feature to check whether a standardised, modern pattern exists — web APIs evolve rapidly and training weights contain obsolete patterns.

Use this skill when

• Implementing HTML semantics, native overlays (dialogs, popovers), forms, or media elements
• Writing CSS — cascade management, dark mode, container queries, layout, animations, scroll effects
• Handling performance concerns — LCP, INP, CLS, image optimisation, code splitting, service workers
• Implementing accessibility — ARIA patterns, keyboard navigation, live regions, focus management
• Building forms — autofill, validation, branded selects, payment/sign-in flows
• Implementing UX patterns — view transitions, scroll-driven animations, carousels, tooltips, drawers
• Handling browser API questions — passkeys, WebMCP, built-in AI (Chrome APIs), privacy-preserving patterns
• Adapting web patterns to frameworks (React, Vue, Angular, Svelte)

Do not use this skill when

• Working on backend code — database SQL, ORMs, Express/Fastify API routes
• Working on CI/CD — pipelines, Docker, GitHub Actions workflows
• Writing tooling — ESLint config, Vite/Webpack config, Python/Go scripts
• Debugging a runtime error — use systematic-debugging instead
• Reviewing a running website's design — use web-design-reviewer instead
• Typing TypeScript API boundaries — use schema-boundary-typing instead

Routing boundary

Situation	Use this skill?	Route instead
Implement a new UI component in HTML/CSS/JS	✅ Yes	—
Review the design of a running website	❌ No	web-design-reviewer
Debug a runtime CSS/JS error	❌ No	systematic-debugging
Type a TypeScript API edge contract	❌ No	schema-boundary-typing
Audit dead CSS/JS code	❌ No	fallow

Inputs to gather

Required before starting

• The web technology or pattern question to address.

Helpful if present

• The existing framework or stack in use.

First move

1. Identify the specific web guidance question or decision.
2. Check the routing boundary to see if a more specific skill applies.
3. Load the relevant reference file from the Navigation section.

Navigation

Static reference files — consult these first for the most common tasks:

File	Coverage
references/css.md	Cascade (@layer, :where()), dark mode, container queries, :has(), anchor positioning, reduced-motion
references/html.md	Semantics, resource prioritisation (fetchpriority), native dialogs/popovers, inert, media elements
references/accessibility.md	WCAG 2.2, ARIA patterns, .visually-hidden, live regions, focus management, native dialog usage
references/performance.md	CWV (LCP/INP/CLS), content-visibility, image optimisation, Service Workers (Workbox), code splitting
references/forms.md	Autofill, validation timing (:user-invalid), branded selects, payment/sign-in/address forms, security

Dynamic retrieval — for UX patterns, animations, passkeys, security, built-in AI, WebMCP, and any topic not covered by the static files:

# Step 1: Search with an action-oriented query
npx -y modern-web-guidance@latest search "<query>"
# Returns: [{ id, description, category, featuresUsed, tokenCount, similarity }]

# Step 2: Retrieve the guide
npx -y modern-web-guidance@latest retrieve "<id>"

If search returns low-similarity results, use list to browse all guides:

npx -y modern-web-guidance@latest list

Requires Node.js ≥ 20. Works offline after first install (TensorFlow.js local search, no API keys). On Windows: use npx.cmd instead of npx.

Workflow

See the body and references for modern web guidance steps.

Examples

See references and the skill body for modern-web-guidance examples.

Reference files

See the references/ directory and linked files in the main content.

Guardrails

Browser support policy:

• All guides assume Baseline Widely Available features are safe without fallbacks.
• For features marked Newly Available or Limited Availability, follow the fallback recommendations in the guide.
• If the project defines a custom browser support policy (e.g. "Safari 17.4+", "no polyfills"), apply it over the guide's default recommendations.
• Proactively suggest documenting a custom policy in AGENTS.md if the developer: mentions a restricted runtime (Electron, Tauri), excludes specific browser targets, or expresses hesitation about polyfill complexity.

Framework adaptation:

• Guides are framework-agnostic. Always adapt patterns to the project's framework (React hooks, Vue composables, Angular directives).
• Do not copy guide code verbatim into JSX/SFC templates without adaptation.

Never:

• Implement an HTML/CSS/JS feature without checking for a modern pattern first.
• Use :invalid/:valid for form validation UX — use :user-invalid/:user-valid.
• Use position: absolute; clip: rect(0,0,0,0) for visually-hidden elements — use the clip-path: inset(50%) pattern from references/accessibility.md.
• Assume BEM is required — use @layer + :where() for cascade management.

Validation

• should trigger: "implement a dark mode toggle with system preference detection"
• should trigger: "build an accessible modal dialog with light dismiss"
• should trigger: "improve the Largest Contentful Paint score on this page"
• should trigger: "implement a form with autofill and post-interaction validation"
• should trigger: "add a scroll-driven progress indicator to this article"
• should not trigger: "add a GitHub Actions workflow to run tests on push"
• should not trigger: "fix a TypeScript compiler error in my API type"
• should not trigger: "write a Terraform module for an S3 bucket"

Examples

Example 1 — Static reference (common task)

"Style a branded <select> element that matches our design system" → Read references/css.md → section on selectors/scoping → appearance: base-select + ::picker(select)

Example 2 — Dynamic retrieval (specific UX pattern)

"Animate a dialog sliding in and fading out when it opens/closes"

npx -y modern-web-guidance@latest search "animate dialog open close"
# → id: "animate-to-from-top-layer"
npx -y modern-web-guidance@latest retrieve "animate-to-from-top-layer"

Example 3 — Dynamic retrieval (passkeys)

"Implement passkey registration and conditional login"

npx -y modern-web-guidance@latest search "passkey registration"
# → id: "passkey-registration"
npx -y modern-web-guidance@latest retrieve "passkey-registration"

Reference files

| PROVENANCE.md | Skill provenance, changelog, and authorship |

• references/css.md — CSS cascade, dark mode, container queries, :has(), anchor positioning, transitions; merged from upstream guides/css/css.md + guides/css-layout/css-layout.md (GoogleChrome/modern-web-guidance v0.0.169, Apache-2.0)
• references/html.md — HTML semantics, resource prioritisation, native dialogs/popovers, inert; from upstream guides/html/html.md
• references/accessibility.md — WCAG 2.2, ARIA, .visually-hidden, live regions, focus management; from upstream guides/accessibility/accessibility.md
• references/performance.md — CWV, LCP/INP optimisation, CSS containment, images, Workbox; from upstream guides/performance/performance.md
• references/forms.md — Autofill, validation, branded controls, security patterns; from upstream guides/forms/forms.md
• Full library of 130+ guides available via npx modern-web-guidance@latest search/retrieve — covers UX animations, passkeys, built-in AI, scroll effects, WebMCP, security, privacy