frontend-engineering

name	frontend-engineering
description	Load when a task's primary output is HTML, CSS, or JS. Provides design pre-flight, codified craft rules, and GATES verification commands for that surface.

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.

PLAN phase — Design Pre-flight

Complete all four steps before writing any code. These are not suggestions; they are the contract for greenfield frontend work.

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:

Goal	Use
Professional / executive SaaS	Linear, Stripe, Vercel
Data-dense / terminal	Raycast, Arc
Minimal / editorial	Notion
Warm / human	Toss

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):

Surface type	Load
Marketing page, landing page, pricing page, acquisition flow	conversion-design
Documentation site, help centre, API reference, technical guide	documentation-design
Dashboard, reporting view, analytics screen, monitoring surface	analytical-design
Article page, editorial page, blog, long-form content page	informational-design
Form flow, component state machines, transitions, interactions	interaction-design
Content strategy — what the surface says and for whom	content-design

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.

State	Wrong	Right
Loading	Generic spinner	Skeleton screen matching the final layout (use spinner only when shape is unknown); add aria-busy="true" and aria-label="Loading <thing>" to the skeleton container
Empty	Blank space	Illustration or icon + label describing the empty condition + primary CTA
Error	Clear prior content; show only the error	Preserve prior content; show inline error message + retry affordance
Partial	No indication more data exists	Pagination or load-more with a record count
Disabled	Hide the element	Render it disabled with aria-disabled="true" and a tooltip or label explaining why
Content	—	The normal loaded state — spec this too so the skeleton shape is known

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.

---

EXECUTE phase — Craft Rules

Avoid the AI Aesthetic

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

Pattern	Why it's a problem	Instead
Purple / indigo everything	Models default to bg-indigo-500 — every generated app looks identical	Use the project's token palette; derive from the named aesthetic reference
Excessive gradients	Add visual noise; clash with most design systems	Flat colour or a single subtle gradient matching the system
Rounded everything (rounded-2xl / border-radius: 16px on all elements)	Ignores the radius hierarchy in real designs — cards, buttons, and inputs each have a distinct radius	Use the --ds-radius-* scale; vary by element type
Generic hero sections	Template-driven layout with no connection to actual content or user need	Content-first layout driven by what the user needs to do
Lorem ipsum placeholder copy	Hides layout problems that real content reveals (wrapping, overflow, long names)	Realistic-length placeholder text that approximates actual content
Oversized equal padding everywhere	Destroys visual hierarchy; wastes screen space	Use the spacing scale; vary padding by component level
Uniform card grids	Ignores information priority and scanning patterns	Purpose-driven layouts — group by relationship, not by grid slot
Shadow-heavy design	Layered shadows compete with content; slow on low-end devices	Use --ds-shadow-sm sparingly; flat or a single elevation level

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

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)

Element	Minimum ratio
Body text	4.5 : 1
Large text (≥ 18 pt or ≥ 14 pt bold)	3 : 1
UI components (borders, focus rings, icons, input outlines)	3 : 1
Placeholder text	4.5 : 1 (counts as text)

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: neither tool currently supports WCAG 2.2 tags — wcag21aa is the current ceiling. Chromium runs fully headless; no DISPLAY environment variable is required.

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 state variants present in the HTML (loading / empty / error / partial / content / disabled) — not just the happy path
• 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

---

Anti-patterns to refuse

Rationalisation	Reality
"Accessibility is a nice-to-have for now"	It is a legal requirement in many jurisdictions (WCAG 2.1 AA is the baseline for EU EAA, ADA, and AODA compliance) and an engineering quality standard — not a feature
"We'll make it responsive later"	Retrofitting responsive design is 3× harder than building it from the start; skip this step only if the output is explicitly fixed-dimension (PPT/PDF)
"This is just a prototype"	Prototypes become production code; the AI aesthetic baked in at prototype stage is the AI aesthetic shipped
"The AI aesthetic is fine for now"	It signals low quality to every reviewer who sees it and anchors the design in a direction that is expensive to undo
"I'll add the empty and error states later"	They will not be added; they are spec ACs, not follow-ons
"Skip the design pre-flight — the spec is clear enough"	Technically correct output with no design sense is the exact failure mode this pre-flight prevents; the spec cannot substitute for token constraints
"Use a spinner, it's simpler"	Spinners produce layout shift and feel slower than skeleton screens; use a skeleton when the content shape is known

---

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 variant missing (empty, error, loading, partial) — 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