frontend-engineering

Skill: frontend-engineering

Load this skill when a task's primary output is HTML, CSS, or JS — a new page, component, slide deck, dashboard, email template, or any standalone web artifact. It carries the design pre-flight requirements (named aesthetic reference, seed token block, state matrix), the craft rules that govern EXECUTE, and the GATES verification commands. It is not needed for incidental HTML edits to an existing surface already covered by a grounded aesthetic reference.

Output rendering

Table — When presenting several items that share the same fields, render a Markdown table. Cap at ~5 columns; beyond that, switch to a per-item detail list. Right-align numeric columns.

Mode selection

Before starting, select the mode that matches the work:

Proceed to the shared pre-flight (PLAN phase) regardless of mode. Mode-specific steps follow the shared pre-flight and the state matrix.

PLAN phase — Shared Pre-flight (all modes)

Complete all four steps before writing any code (create/retrofit) or running any gates (audit/verify). These are the shared foundation for all modes.

1. Named aesthetic reference

State a named product reference — not an adjective. The model has learned visual vocabulary from extensively documented products; vague adjectives produce the purple-gradient default (Tailwind's bg-indigo-500 saturated training data, so "nice", "clean", "modern" all converge there).

Canonical reference set:

Name the reference in the spec: Aesthetic reference: Linear (professional SaaS — dark surface, high contrast, no gradients). If the target must match an existing user-provided theme (e.g. a PPT brand), describe its key token values instead.

1b. Genre routing (T2 — requires experience-design pack)

After naming the aesthetic reference, route to the XD discipline skill that matches your surface's primary purpose. These skills add surface-specific IA, structure, and conversion principles on top of the generic design pre-flight.

Check availability: the experience-design pack is installed if skill conversion-design appears in your available skills. If absent, record a named skip in the spec — XD genre routing: skipped (experience-design pack absent) — and proceed to step 2. A named skip is not a failure; it is honest accounting.

Load the skill that matches your surface (name it by name in the spec):

Load the matched skill inline before writing code. Record the result in the spec as either XD genre routing: <skill-name> loaded or XD genre routing: skipped (experience-design pack absent).

2. Seed token block

Provide a CSS custom properties block before writing any HTML. The model selects from var(--ds-color-primary) rather than fabricating #5e6ad2 per session — token-seeding is the single strongest lever for visual consistency.

Three-tier architecture (one-way dependency):

Primitive  →  Semantic  →  Component
(raw hex)      (role)       (usage)

Only the semantic layer goes in the seed block; primitives are defined once at the top of the CSS file and referenced by semantics.

Minimum viable property set (--ds- prefix for namespace clarity):

:root {
  /* Color roles — semantic, not raw hex */
  --ds-color-surface:      #ffffff;
  --ds-color-surface-alt:  #f8fafc;
  --ds-color-on-surface:   #1a202c;
  --ds-color-on-surface-2: rgba(0, 0, 0, 0.60);
  --ds-color-primary:      #5e6ad2;
  --ds-color-on-primary:   #ffffff;
  --ds-color-error:        #dc2626;
  --ds-color-on-error:     #ffffff;
  --ds-color-outline:      rgba(0, 0, 0, 0.12);

  /* Spacing — 4 px base, 8-step scale */
  --ds-space-px: 2px;
  --ds-space-1:  4px;
  --ds-space-2:  8px;
  --ds-space-3:  12px;
  --ds-space-4:  16px;
  --ds-space-5:  24px;
  --ds-space-6:  32px;
  --ds-space-7:  48px;
  --ds-space-8:  64px;

  /* Type scale */
  --ds-text-sm:   0.75rem;
  --ds-text-base: 0.875rem;
  --ds-text-lg:   1rem;
  --ds-text-xl:   1.125rem;
  --ds-text-2xl:  1.25rem;
  --ds-font-regular: 400;
  --ds-font-medium:  500;
  --ds-font-bold:    600;
  --ds-leading-tight:  1.25;
  --ds-leading-normal: 1.5;
  --ds-leading-loose:  1.75;

  /* Radius */
  --ds-radius-sm: 4px;
  --ds-radius-md: 8px;
  --ds-radius-lg: 12px;
  --ds-radius-full: 9999px;

  /* Shadow */
  --ds-shadow-sm: 0 1px 2px rgba(0,0,0,0.06);
  --ds-shadow-md: 0 4px 8px rgba(0,0,0,0.08);
  --ds-shadow-lg: 0 8px 24px rgba(0,0,0,0.10);

  /* Motion */
  --ds-duration-quick:    120ms;
  --ds-duration-moderate: 200ms;
  --ds-duration-gentle:   300ms;
  --ds-ease-standard:     cubic-bezier(0.4, 0, 0.2, 1);
  --ds-ease-decelerate:   cubic-bezier(0, 0, 0.2, 1);
}

Print / PPT token block

When the output targets a PPT slide or PDF export, add this block and use pt for typographic values:

@page {
  size: 960px 540px; /* 16:9 slide — standard widescreen */
  margin: 0;
}

* {
  -webkit-print-color-adjust: exact;
  print-color-adjust: exact; /* preserve background fills */
}

@media print {
  :root {
    --ds-color-surface:    #ffffff;
    --ds-color-on-surface: #000000;
    --ds-shadow-sm: none;
    --ds-shadow-md: none;
    --ds-shadow-lg: none;
  }

  .slide            { page-break-after: always; }
  h2, h3, figure,
  table, blockquote { page-break-inside: avoid; }
}

Print safety: box-shadow and text-shadow are unreliable across renderers (Chrome/WeasyPrint differ) — use --ds-shadow-*: none in the print override and rely on borders for separation instead. Avoid Tailwind responsive variants (sm:, md:) for fixed-dimension artifacts.

3. State matrix

Enumerate all states for every async component as a table in the spec. LLMs are trained predominantly on happy-path code; they will not generate missing-state branches without explicit enumeration. A 2025 study of 50 AI-generated dashboards found 92% had no empty state and 78% had no error state.

The canonical 18-state set for this skill, aligned with the XD quality-floor:

Skeleton vs spinner rule: use a skeleton when the content shape is predictable (table rows, card grids, profile cards). The skeleton must match the final layout so there is no layout shift on load. Use a spinner only when shape is genuinely unknown.

Not all states apply to every surface — omit states that are genuinely inapplicable and note why in the spec.

Mode: create

Use when building a new surface or significant new component.

Step 0. Page/screen contract (required before significant UI code)

Before writing HTML for a new page or significant surface, fill the page/screen contract. This contract is proportional to risk and scope — a new route, a key onboarding surface, or a feature-gating screen warrants the full 12-field contract. A single new form field, a tooltip, or a minor component variant does not.

Page/screen contract template (12 fields):

Record the completed contract in the spec before writing HTML.

Steps 1–3. Proceed through the shared PLAN phase pre-flight

Run steps 1, 1b, 2, and 3 from the shared pre-flight above (aesthetic reference, genre routing, seed tokens, state matrix).

EXECUTE and GATES

Proceed to the EXECUTE phase (Craft Rules) and GATES phase below. Produce an evidence manifest at completion.

Mode: retrofit

Use when improving or extending an existing surface without building from scratch.

Step 1. Brownfield inspection checklist

Before touching any code, run this inspection against the existing surface. Record findings for each item:

Step 2. Proceed through the shared PLAN phase pre-flight

Run steps 1, 1b, 2, and 3 from the shared pre-flight (aesthetic reference, genre routing, seed tokens, state matrix). For retrofit work, focus the state matrix on states that are absent or broken — not a full re-enumeration unless the surface is substantially rebuilt.

EXECUTE and GATES

Proceed to the EXECUTE phase (Craft Rules) and GATES phase below. Produce an evidence manifest at completion.

Mode: audit

Use when reviewing an existing surface without writing code. The output is a structured audit report, not code.

Audit procedure

1. Run the state matrix audit. Compare the surface against all 18 states in the state matrix. For each applicable state, mark: Covered / Absent / Broken. Note the specific issue for Absent and Broken.
2. Run the accessibility audit. Check against WCAG 2.2 AA. Use the GATES accessibility tools (pa11y or axe-core). Note which WCAG 2.2 success criteria require manual verification because tooling caps at wcag21aa.
3. Check the CWV targets. Measure or estimate LCP ≤2.5s / INP ≤200ms / CLS ≤0.1 at p75 (mobile and desktop separately where field data exists). Note any category over budget.
4. Run the brownfield inspection checklist. Use the same 6-item checklist from retrofit mode.

Audit report format

Return findings as a prioritised list with severity (Blocker / Major / Minor / Note). Each finding maps to the state, criterion, or checklist item it violates, with one concrete recommendation.

Record findings in the evidence manifest under known exceptions and unverified items.

Mode: verify

Use when a completed surface needs gates run and an evidence manifest generated.

Verification procedure

Run the full GATES suite in order:

1. Structural HTML validation (GATES phase step 1)
2. Accessibility audit (GATES phase step 2) — note that WCAG 2.2 AA is our declared baseline; the tooling caps at wcag21aa; two success criteria require manual verification: 2.4.11 Focus Appearance and 2.5.8 Target Size Minimum
3. CSS token enforcement (GATES phase step 3, if stylelint is configured)
4. Visual QA checklist (GATES phase step 4) — confirm all 18 applicable states are present

After running all four gates, generate the evidence manifest (see Evidence manifest section below) with the results.

EXECUTE phase — Craft Rules

Avoid the AI Aesthetic

AI-generated UI has recognisable failure patterns. Refuse all of them:

HTML element selection rules

Rules are in WRONG → RIGHT form. "Use semantic HTML" is not a rule; the specific forms below are.

Interactive elements

• WRONG: <div onclick="…">, <span onclick="…"> / RIGHT: <button type="button"> for any action
• WRONG: <a href="#" onclick="…"> for actions / RIGHT: <a href="…"> for navigation only; <button> for actions — never cross them
• WRONG: <div role="button"> with no keyboard handler / RIGHT: <button> — it receives keyboard, focus, and activation natively; avoid role="button" on a non-button element unless a framework absolutely requires it, and then you must add tabindex="0" and onkeydown handlers for both Enter and Space

Landmark elements

• One <main> per page, no exceptions
• One <h1> per page
• Heading levels are sequential — never skip (h1 → h3 is wrong; heading level = outline position, never visual size)
• <section> only when it has a heading as its first child; otherwise use <div>
• Multiple <nav> elements require aria-label distinguishing them: aria-label="Primary", aria-label="Breadcrumb"
• <article> for self-contained distributable content; <aside> for tangentially related content

Forms

• WRONG: <input placeholder="Email"> with no label / RIGHT: <label for="email">Email</label><input id="email"> or aria-labelledby; placeholder is not a label — it fails WCAG 1.3.1 and disappears on input
• WRONG: aria-describedby pointing to an element not yet in the DOM / RIGHT: place the target element in the DOM before the input receives focus; inject empty containers on page load
• WRONG: ungrouped checkboxes/radios / RIGHT: <fieldset><legend>…</legend>…</fieldset> for every checkbox or radio group
• autocomplete attribute is required on personal-data fields: name, email, tel, current-password, new-password, address fields

Images and media

• WRONG: alt="image", alt="photo of a dog" / RIGHT: descriptive text without "image of" / "photo of" prefixes; alt="" for decorative images; max ~150 chars — use <figcaption> for longer descriptions
• WRONG: <img> for decorative backgrounds / RIGHT: CSS background-image — the element is removed from the accessibility tree automatically
• SVG used as a meaningful image: add role="img" and a <title> as the first child
• SVG that is decorative: aria-hidden="true"

Content

• WRONG: lorem ipsum placeholder text / RIGHT: realistic-length placeholder text that approximates what real content looks like — long names, multi-line descriptions, edge-case values

CSS rules

• WRONG: color: #5e6ad2, background: #f8fafc, margin: 13px / RIGHT: all colour and spacing values via var(--ds-*) — no hardcoded hex, rgb, hsl, or magic pixel values
• WRONG: z-index: 9999 / RIGHT: define a named z-index scale: --z-base: 0; --z-overlay: 100; --z-modal: 200; --z-toast: 300 — use named custom properties only
• WRONG: line-height: 24px / RIGHT: unitless — line-height: var(--ds-leading-normal) or line-height: 1.5
• WRONG: #nav {}, ul.nav {} / RIGHT: class selectors only; no ID selectors; no qualified selectors
• WRONG: selector depth > 3 levels / RIGHT: max 3 levels of nesting
• WRONG: tabindex="2", tabindex="5" (positive values) / RIGHT: tabindex="0" to enter tab order; tabindex="-1" for programmatic focus targets only; never positive values — they disrupt the natural tab sequence
• WRONG: .btn:hover { … } with no focus style / RIGHT: every :hover rule has a matching :focus or :focus-visible rule
• WRONG: outline: none / RIGHT: replace with a visible alternative — outline: 2px solid var(--ds-color-primary); outline-offset: 2px or a box-shadow equivalent

Accessibility rules

Default baseline: WCAG 2.2 AA. WCAG 2.2 AA is our baseline — it exceeds the WCAG 2.1 minimum currently cited by EU EAA, ADA, and AODA. Two success criteria are new in 2.2 and require manual verification because automated tooling (pa11y/axe-core) caps at wcag21aa: 2.4.11 Focus Appearance (visible focus indicator with sufficient size and contrast) and 2.5.8 Target Size Minimum (interactive targets ≥24×24 CSS pixels). Mark these explicitly in the evidence manifest under a11y result.

Browser policy: Baseline Widely Available. Target only features in the Baseline Widely Available set (features shipping in all major browsers for at least 30 months). Check baseline status at web.dev/baseline before using any feature not in the baseline set.

ARIA discipline

First Rule of ARIA: if a native HTML element provides the semantics and behaviour, use it — do not bolt ARIA onto a generic element.

• WRONG: <div role="button" onclick="…"> — no keyboard, no activation, no inherited states / RIGHT: <button>
• WRONG: aria-label="Submit" on a <button>Submit</button> with visible text / RIGHT: omit aria-label — it overrides the visible label and breaks voice control ("Submit" no longer activates the button by name in Dragon NaturallySpeaking)
• WRONG: <input aria-label="Email" placeholder="Email"> when a visible label exists / RIGHT: use the <label> and omit the redundant aria-label

Dynamic ARIA state must update — these are not static attributes:

• aria-expanded="false" on a closed accordion must flip to "true" when open
• aria-selected must update as tabs are navigated
• aria-sort must update as columns are sorted
• Setting them once in the HTML and never updating them with JS is wrong

aria-live regions:

• The container must be in the DOM before content is injected into it — add it empty on page load and update its text content to trigger the announcement; injecting a live region and populating it simultaneously causes the announcement to be dropped silently in most screen readers
• aria-live="polite" for informational updates (toast success, search result counts)
• aria-live="assertive" + aria-atomic="true" for errors and time-critical alerts (session timeout, form validation summary)

WCAG contrast floor (WCAG 1.4.3 / 1.4.11)

Verify contrast at derivation time — never eyeball it.

Reduced motion

Every animation, transition, and transform must be guarded. Default to no motion; enable only when the user has not requested reduced motion:

/* Default: no motion */
.element {
  transition: none;
  animation: none;
}

/* Motion only when the user permits */
@media (prefers-reduced-motion: no-preference) {
  .element {
    transition: opacity var(--ds-duration-moderate) var(--ds-ease-standard);
  }
}

Properties to guard: animation, transition, transform (slides, scales, rotates), scroll-behavior: smooth. Colour and opacity changes that carry meaning do not need guarding — only motion.

Modal / dialog keyboard pattern (W3C APG)

role="dialog" + aria-modal="true" + aria-labelledby="<title-id>"
On open:   move focus to first focusable element inside the dialog
Tab:       cycle forward through focusable elements inside only (trap)
Shift+Tab: cycle backward (trap)
Escape:    close the dialog
On close:  return focus to the element that opened the dialog

Tabs keyboard pattern (W3C APG)

Tab:         moves focus into the tablist, then out to the tabpanel — never between tabs
Left/Right:  navigate between tabs (wrapping); activate on focus (auto-activation)
Home/End:    jump to first / last tab

Programmatic focus — when it is required

Move focus programmatically when:

• A modal opens (to first focusable element inside) or closes (back to invoking element)
• A SPA route changes (to the page <h1> or a skip-nav landmark with tabindex="-1")
• An inline error appears after form submit (to first invalid field or error summary)
• Async content is inserted and the user needs to interact with it immediately

GATES phase — Verification

Run these after implementation, before review. Record actual command output — do not assert on internal state.

1. Structural HTML validation (no browser required)

npx html-validate --preset standard,a11y --max-warnings 0 <file.html>

Catches without a browser: landmark structure (one <main>, unique landmarks), heading hierarchy (no skipped levels), label associations, ARIA role validity, alt attribute presence, and WCAG H-technique violations. Exits non-zero on any error.

2. Accessibility audit (requires Chromium — runs headless, no display server)

Either tool works; pa11y is lighter, axe-core has more rules:

# pa11y — local files via file:// path
npx pa11y "file:///$(pwd)/file.html" --standard WCAG2AA --reporter cli

# axe-core/cli — URL or file, CI-safe Chromium flags
npx axe "file:///$(pwd)/file.html" \
  --tags wcag21aa \
  --chrome-options="no-sandbox,disable-setuid-sandbox,disable-dev-shm-usage"

Note: tooling currently caps at wcag21aa — WCAG 2.2 AA is our declared baseline (it exceeds the wcag21aa minimum). Two WCAG 2.2-only success criteria require manual verification: 2.4.11 Focus Appearance and 2.5.8 Target Size Minimum. Record the manual-check outcome in the evidence manifest under a11y result.

3. CSS token enforcement (optional — run if stylelint is already configured)

{
  "plugins": ["stylelint-declaration-strict-value"],
  "rules": {
    "scale-unlimited/declaration-strict-value": [
      ["color", "background-color", "border-color", "font-size"],
      {
        "ignoreValues": ["inherit", "transparent", "currentColor"],
        "message": "Use design tokens (var(--ds-*)) — no hardcoded values"
      }
    ]
  }
}

Install: npm install --save-dev stylelint stylelint-declaration-strict-value

4. Visual QA checklist (agent-executable, no tooling)

• All applicable states from the 18-state matrix are present in the HTML — not just the happy path; check each applicable state by reading the HTML
• No hardcoded colour or spacing values outside the token-definition block — grep: grep -E "#[0-9a-fA-F]{3,6}|rgba?\(|hsl\(|[0-9]+px" <file.css> should return only the :root / primitive token-definition block, no other hex, rgb, or px values
• Print output correct: if PPT/PDF context, open in browser and trigger print preview — check slide boundaries, colour preservation, no overflow
• Screenshot taken and observed: Playwright headless (page.screenshot()) or browser devtools screenshot — assert on what you see, not on internal state

Performance targets

Core Web Vitals (CWV)

Targets at p75, evaluated separately for mobile and desktop where field data exists:

Measure using Lighthouse, Chrome DevTools Performance panel, or WebPageTest.

Asset budgets

Enforce these per route. The seven asset budget categories to track are: JS budget (JavaScript parse+execute per route), images budget (total image payload per route), fonts (web font files transferred), third-party scripts (analytics, tags, widgets), hydration (client-side hydration cost for SSR/islands), route-level loading (per-route code-split chunks), and long tasks (main-thread tasks blocking >50ms).

Evidence manifest

FE cannot claim completion (create or retrofit) or a passing gate run (verify) without an evidence manifest. The manifest is a structured record of what was tested and what was found.

Required fields (all 11 must be present):

Conditional public-surface guidance

Applies only when the surface will be publicly indexed (marketing pages, documentation, product landing pages). Skip for internal tools, authenticated dashboards, and surfaces behind login.

For publicly indexed surfaces, add these items to the spec and verify them before merge:

• Metadata: <title> (60 chars max), <meta name="description"> (155 chars max), Open Graph tags (og:title, og:description, og:image) for link previews
• Canonical URLs: <link rel="canonical" href="…"> on every public page; avoid duplicate content from trailing slashes, www vs non-www, or protocol variants
• Sitemaps: ensure the route is included in the sitemap or explicitly excluded; no orphaned pages
• Structured data: appropriate schema.org type for the surface (Article, Product, FAQPage, HowTo) implemented as JSON-LD; validate at schema.org/validator
• Search indexing intent: confirm robots meta or X-Robots-Tag header is set correctly — index, follow for pages that should rank; noindex for pagination, internal search results, thin pages

Multi-surface shell contract

Applies when building or reviewing a product with multiple web surfaces (e.g. marketing site + web app + documentation site).

Multi-surface products must maintain coherence across surfaces. Apply these constraints regardless of which surface you are currently building:

• Shared tokens: all surfaces draw from the same design token contract (same --ds-* custom property names, same primitive values, same semantic assignments). A surface that redefines shared tokens independently forks the product's visual identity.
• Navigation patterns: primary navigation, breadcrumb, and footer patterns must be consistent across surfaces — same structure, same interaction model, same terminology for shared destinations.
• Consistent product terminology: the names of features, entities, and actions must be the same across surfaces. Maintain a terminology list in the project and use it before naming anything.

Anti-patterns to refuse

Red flags

A reviewer should treat any of these as a blocker:

• Inline style="…" attributes or arbitrary pixel values not on the spacing scale
• Any state from the applicable 18-state matrix missing from the implementation — check by reading the HTML, not by reasoning about the code
• outline: none or outline: 0 with no visible replacement focus style
• Color used as the sole state indicator (red/green without accompanying text, icon, or pattern)
• Generic AI aesthetic visible in the output (purple gradients, equal oversized padding, border-radius: 16px on every element, shadow on every card)
• aria-expanded, aria-selected, or aria-sort set once and never updated
• A <div onclick> where a <button> would serve
• FE completion claimed without an evidence manifest
• Multi-surface product with inconsistent shared tokens, navigation patterns, or product terminology across surfaces