UI Engineering

Overview

Construct UI components with disciplined architecture, not improvised markup.

Core principle: Every component decision -- layout strategy, state ownership, accessibility posture, responsive behavior -- follows established patterns. The UX Patterns skill tells you WHAT to build. This skill tells you HOW to build it with structural integrity.

PREREQUISITE: Invoke godmode:ux-patterns first to identify applicable patterns and token values. This skill assumes tokens and patterns are already established.

When to Use

Mandatory when:

• Constructing any frontend component
• Selecting between Grid and Flexbox
• Deciding where state should live
• Engineering responsive breakpoint behavior
• Building forms, data visualizations, navigation, or overlays
• Introducing motion or transitions

Sequenced after:

• Design tokens are established (ux-patterns)
• Target UI pattern is identified (ux-patterns)

The Prime Directive

NO COMPONENT WITHOUT STRUCTURE, STATES, AND ACCESSIBILITY DEFINED FIRST

Before writing component code, establish: semantic structure (correct HTML elements), all visual states (empty, loading, error, populated, disabled), and accessibility requirements (ARIA attributes, keyboard interaction, contrast ratios).

Component Architecture

Composition Over Configuration

Assemble components from smaller, composable units rather than monolithic prop-heavy blocks.

Pre-Implementation Checklist

Before writing any component:

1. Semantics -- Which HTML element is correct? (button not div onClick, nav not div className="nav")
2. Props -- What is the minimal surface area? Can it be composed instead of configured?
3. States -- Default, hover, focus, active, disabled, loading, error, empty
4. Variants -- What visual variations are needed? (primary, secondary, ghost, destructive)
5. Sizes -- What size tiers exist? (sm, md, lg -- maximum 3-4)
6. Responsive -- How does it transform at each breakpoint?
7. Accessibility -- ARIA roles, keyboard navigation paths, screen reader announcements

Layout Strategy Selection

digraph layout_choice {
    rankdir=TB;
    q1 [label="What is being\narranged?", shape=diamond];
    q2 [label="Single axis or\ntwo axes?", shape=diamond];
    q3 [label="Are items\nuniform in size?", shape=diamond];

    grid [label="Use CSS Grid\ngrid-template-columns\ngrid-template-rows", shape=box];
    flex [label="Use Flexbox\nflex-direction\njustify/align", shape=box];
    grid_auto [label="Use CSS Grid\nauto-fill/auto-fit\nminmax()", shape=box];

    q1 -> q2;
    q2 -> grid [label="two axes\n(rows AND columns)"];
    q2 -> q3 [label="single axis\n(row OR column)"];
    q3 -> grid_auto [label="yes\n(uniform cards)"];
    q3 -> flex [label="no\n(nav items,\nform row)"];
}

CSS Grid -- Appropriate When

• Page-level scaffolding (sidebar + main content + aside)
• Uniform card grids
• Dashboard arrangements (metric tiles, chart regions)
• Any layout requiring two-dimensional control
• Cross-row and cross-column alignment

Example patterns:

/* Self-adjusting card grid */
.card-grid {
  display: grid;
  grid-template-columns: repeat(auto-fill, minmax(280px, 1fr));
  gap: var(--space-6);
}

/* Dashboard scaffold */
.dashboard {
  display: grid;
  grid-template-columns: 1fr 1fr 1fr;
  grid-template-rows: auto 1fr;
  gap: var(--space-6);
}
.dashboard .wide-chart { grid-column: span 2; }

Flexbox -- Appropriate When

• Navigation items along a single row
• Form input + button inline grouping
• Centering content within a container
• Distributing variable-width items along one axis

Example patterns:

/* Navigation row */
.nav-row {
  display: flex;
  align-items: center;
  gap: var(--space-4);
}

/* Inline form group */
.inline-group {
  display: flex;
  align-items: flex-end;
  gap: var(--space-3);
}
.inline-group .input-field { flex: 1; }

State Ownership Strategy

digraph state_ownership {
    rankdir=TB;
    q1 [label="Where does this\nstate belong?", shape=diamond];
    q2 [label="Consumed by\nmultiple components?", shape=diamond];
    q3 [label="Server-originated\nor client-only?", shape=diamond];
    q4 [label="Prop-drilling\nexceeds 3 levels?", shape=diamond];

    local [label="Component-local state\nuseState / ref", shape=box];
    server [label="Server state manager\nTanStack Query / SWR", shape=box];
    context [label="Context / Provider\nReact Context / provide-inject", shape=box];
    global [label="Global store\nZustand / Pinia / Signals", shape=box];

    q1 -> q2;
    q2 -> local [label="no\n(isolated component)"];
    q2 -> q3 [label="yes"];
    q3 -> server [label="server-originated\n(API responses,\ncached data)"];
    q3 -> q4 [label="client-only\n(UI flags,\npreferences)"];
    q4 -> context [label="no\n(2-3 levels)"];
    q4 -> global [label="yes\n(application-wide)"];
}

Governing principles:

1. Begin with local state. Elevate only when evidence demands it.
2. Server data is NOT client state. Manage it with a dedicated server-state library.
3. Context is for dependency injection (themes, auth context), not for high-frequency updates.
4. Global stores are a last resort, not a starting point.

Responsive Design Methodology

Mobile-First Progression

Write mobile styles as the baseline, then layer complexity at wider breakpoints.

/* Mobile baseline */
.wrapper {
  padding: var(--space-4);
}

/* Tablet tier */
@media (min-width: 768px) {
  .wrapper {
    padding: var(--space-6);
    max-width: 768px;
    margin: 0 auto;
  }
}

/* Desktop tier */
@media (min-width: 1024px) {
  .wrapper {
    padding: var(--space-8);
    max-width: 1280px;
  }
}

Responsive Adaptation Reference

Element	Mobile	Tablet	Desktop
Navigation	Hamburger or bottom sheet	Tab bar or collapsed sidebar	Expanded sidebar
Card grid	Single column	Two columns	Three to four columns
Data table	Stacked card view or horizontal scroll	Full table, fewer columns	Complete table
Sidebar + Main	Main only; sidebar in drawer	Icon-only sidebar	Fully expanded sidebar
Form	Single column, inputs stretch full width	Single column, max-width 560px	Two columns for paired fields
Modal	Full-screen sheet	Centered, 80% viewport width	Centered, max-width 480px
Hero	Stacked (image below headline)	Stacked, larger type	Side-by-side

Accessibility Standards

Every Interactive Element

• Reachable via Tab key
• Focus ring visible (:focus-visible, 2px outline minimum)
• Activatable via Enter/Space (buttons) or Enter (links)
• Possesses an accessible name (visible text, aria-label, or aria-labelledby)
• Disabled state removes from tab order or applies aria-disabled
• Touch target meets 44x44px minimum on mobile

Forms

• Every input has a visible <label> (placeholder alone is insufficient)
• Required fields indicated (asterisk plus aria-required="true")
• Error messages connected to their input (aria-describedby)
• Error indication uses more than color alone (icon plus text)
• Submission outcomes announced to screen readers (aria-live)

Images and Icons

• Meaningful images carry descriptive alt text
• Decorative images carry alt="" or aria-hidden="true"
• Icon-only buttons carry aria-label
• SVG icons use role="img" with aria-label or aria-hidden="true"

Color and Contrast

• Normal text meets 4.5:1 contrast ratio (3:1 for large text: 18px+ bold or 24px+)
• Information is never conveyed by color alone (supplement with icons, patterns, text)
• Motion respects prefers-reduced-motion
• Color scheme respects prefers-color-scheme if dark mode is offered

Animation and Motion

When to Animate

• State transitions: Hover, focus, expand/collapse, reveal/hide
• Feedback signals: Success, error, loading progress
• Spatial cues: Communicating where content originated or departed

When NOT to Animate

• Decoration with no functional purpose
• Durations exceeding 300ms for UI transitions
• Motion that blocks user interaction (forced wait)
• Continuous movement without user control

Timing Reference

Interaction	Duration	Easing
Hover response	150ms	ease
Button press	100ms	ease-out
Modal entrance	200ms	ease-out
Modal exit	150ms	ease-in
Drawer slide	250ms	ease-out
Fade entrance	200ms	ease
Page transition	200-300ms	ease-in-out

Respect User Motion Preferences

@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after {
    animation-duration: 0.01ms !important;
    transition-duration: 0.01ms !important;
  }
}

Form Architecture

Validation Experience

1. Do not validate on every keystroke. Validate on blur (field exit) or on form submission.
2. Display errors inline beneath the offending field, not in a summary banner.
3. Error text displaces helper text -- never display both simultaneously.
4. Error indication uses color plus icon -- never color alone (accessibility).
5. Retain valid-field indicators (checkmark or green border) as the user progresses.
6. Disable the submit button during submission and show a loading indicator.

Field Layout Guidelines

• Labels above inputs (never beside, never placeholder-only)
• Related fields grouped with <fieldset> and <legend>
• Required fields marked with asterisk AND (required) for screen readers
• Optional fields may display "(optional)" instead
• Maximum form width: 480-560px single-column
• Submit button left-aligned, not centered (exception: inside modals)

Rendering Performance

Critical Path Optimization

1. Above-fold content loads first. Defer below-fold assets and scripts.
2. Prefer system fonts for body text unless branding mandates a web font. Web fonts introduce layout shift.
3. Image discipline: Apply width/height attributes (prevents CLS), loading="lazy" for below-fold images, srcset for responsive delivery.
4. Bundle awareness: Every imported library is a cost. Measure bundle impact before adding dependencies.

Lazy Loading Strategy

• Below-fold images: loading="lazy"
• Route modules: dynamic import / code splitting
• Heavy components (charts, rich editors): load on interaction or visibility
• Never lazy-load above-fold content

Common Mistakes

Mistake	Correction
div for everything	Use semantic HTML (button, nav, main, section, article)
Placeholder as sole label	Always provide visible <label> elements
Pixel literals everywhere	Use token variables from the design system
Fixed widths on containers	Use max-width + percentage or auto margins
Removing focus outlines	Style :focus-visible instead of eliminating outline
Click handlers on divs	Use <button> or <a> for interactive elements
Margins for layout spacing	Use gap with Grid or Flexbox
Importing entire icon libraries	Import individual icons; enable tree-shaking
Only implementing the happy path	Design all states before building the happy path
Testing only on desktop	Test mobile-first, verify desktop afterward

Integration

Prerequisite:

• godmode:ux-patterns -- Tokens and patterns must be established first

Complementary skills:

• godmode:design-integration -- When operating within an established design system
• godmode:test-first -- Component tests follow test-first methodology

Supporting files:

• component-patterns.md -- Reusable component blueprints with accessibility built in