# DESIGN.md · Microsoft Fluent 2 (DS-005)

> Written for AI coding agents. Values marked **OFFICIAL** come from the documented source named below; values marked **ESTIMATED** were measured by us from our own screenshots and are approximate. Never treat ESTIMATED values as official brand tokens.

## Overview
- Type: Design System · Category: C. Design Systems
- Platforms: Web, React, Windows, iOS, Android, Figma
- Color theme: Monochrome light · Layout pattern: Documentation · Mood: Soft, approachable, productive
- License / terms: MIT (Fluent UI code and @fluentui/react-theme, © Microsoft Corp.); fonts and icons referenced in Fluent UI React fall under Microsoft’s assets licence per the README (Segoe UI is not open); the Fluent UI System Icons repo itself is MIT
- Industry: Platform design system
- Patterns: Docs sidebar, Live component demo

Microsoft’s cross-platform design system behind Microsoft 365. Fluent UI React v9 ships alias tokens (colorNeutralBackground1, colorBrandBackground…) for web light/dark, a 10–68 px type ramp in Segoe UI, a six-step shadow ramp (shadow2–shadow64), a 2–40 px radius scale and 50–500 ms motion. fluent2.design was unreachable from our box, so evidence comes from the official Storybook and npm package.

### Analysis (ours)
Fluent 2 is the design language of Microsoft 365, Teams and Windows, and Fluent UI React v9 is its web implementation. Instead of Carbon’s flat layers it uses soft elevation: a six-step shadow ramp (shadow2 to shadow64) that pairs an ambient 2–8 px blur with a key shadow, a radius scale whose everyday steps are 2, 4, 6 and 8 px, and a neutral palette that steps from colorNeutralBackground1 to 6. Colour is split into a brand ramp (Communication Blue, #0f6cbd as colorBrandBackground in web light) and neutral and status palettes, all exposed as alias tokens you consume through FluentProvider and the tokens object, so a theme is a plain object you can swap. The type ramp runs from caption2 (10/14 px) to display (68/92 px), with 14/20 px body1 as the default, in Segoe UI, which is not openly licensed: on the web, Fluent falls back to the system stack. Lessons: expose themes as data, keep one brand ramp and derive hover/pressed from it, use elevation sparingly and consistently, and plan a font fallback when the brand font cannot be shipped.

### Best practices (ours)
- Wrap the app in FluentProvider and read colours, spacing and type from the tokens object; never hard-code HEX values.
- Derive a custom brand from a 16-step brand ramp so hover, pressed and selected states stay consistent.
- Use the shadow ramp sparingly: shadow2–4 for cards at rest, shadow16–28 for flyouts, shadow64 for dialogs.
- Keep body text at body1 (14/20 px) and use the type styles (typographyStyles) rather than setting sizes manually.
- Plan a font fallback: Segoe UI can’t be bundled freely, so test the system stack on macOS and Linux.
- Use durationFast–durationNormal (150–200 ms) for state changes and keep slow durations for large surfaces.
- Respect colorStrokeFocus2 focus rings and high-contrast mode; don’t remove outlines.

### Deep analysis
Values marked OFFICIAL come from the cited official docs page or the official npm token package installed 2026-10-25 (UTC+8) (@carbon/themes 11.78.0, @carbon/type 11.70.0, @carbon/layout 11.62.0, @carbon/motion 11.55.0, @primer/primitives 11.10.0, @fluentui/react-theme 9.2.2). ESTIMATED values were computed or observed by us that day (WCAG 2 contrast ratios, what our captures show). OURS is our interpretation.

#### Purpose and audience (OFFICIAL) · source: https://github.com/microsoft/fluentui
- “Fluent UI web represents a collection of utilities, React components, and web components for building web applications” (microsoft/fluentui README). Fluent 2 is the design language behind Microsoft 365, Teams and Windows.
- Licence: “MIT license … Usage of the fonts and icons referenced in Fluent UI React is subject to the terms of the assets license agreement.”
- fluent2.design (design guidance site) could not be reached from our box on 2026-10-25 (TLS connection closed), so this entry cites the official Storybook, GitHub README and npm package instead.

#### Theme and colour model (OFFICIAL) · source: https://storybooks.fluentui.dev/react/?path=/docs/theme-colors--docs
- Themes are plain objects (webLightTheme, webDarkTheme, teamsLightTheme, teamsDarkTheme, teamsHighContrastTheme) passed to FluentProvider; components read alias tokens via the tokens object.
- Alias tokens are grouped as neutral (backgrounds 1–6, foregrounds 1–4, strokes), brand (from a 16-step brand ramp) and palette/status (red, green, yellow…).
- Brand in web light: colorBrandBackground #0f6cbd, hover #115ea3, pressed #0c3b5e; text on brand #ffffff.
- The Storybook Theme section documents Border Radii, Colors, Fonts, Shadows, Spacing, Stroke Widths and Typography and includes a Theme Designer for custom brand ramps.

#### Typography, elevation and motion (OFFICIAL) · source: https://www.npmjs.com/package/@fluentui/react-theme
- Type ramp from caption2 (10/14 px) to display (68/92 px); body1 14/20 px is the default; titles and subtitles are Semibold.
- Font stack: Segoe UI, Segoe UI Web (West European), -apple-system, BlinkMacSystemFont, Roboto, Helvetica Neue, sans-serif.
- Shadow ramp shadow2…shadow64: each pairs an ambient 2–8 px blur at 12% black with a key shadow at 14% black.
- Durations 50–500 ms; curves include easyEase (0.33,0,0.67,1) and decelerateMid (0,0,0,1).

#### Components and states (OFFICIAL) · source: https://storybooks.fluentui.dev/react/?path=/docs/components-button-button--docs
- Button appearances: secondary (default), primary, outline, subtle and transparent; sizes small, medium, large; shapes rounded, circular, square; icon-only and icon-position options; disabled and disabledFocusable states (Fluent UI React v9 Button props).
- Each token has rest, hover, pressed and selected variants (e.g. colorNeutralBackground1Hover #f5f5f5, Pressed #e0e0e0).
- Focus uses a two-colour ring (colorStrokeFocus1/2) so it stays visible on any background.

#### Contrast checks we ran (ESTIMATED) · source: https://www.npmjs.com/package/@fluentui/react-theme
- colorNeutralForeground1 on Background1: light 15.52:1, dark 14.55:1.
- colorNeutralForeground3 (tertiary text) on white: 6.19:1.
- White on colorBrandBackground #0f6cbd: 5.38:1.
- colorNeutralForegroundDisabled #bdbdbd on white: 1.88:1; disabled text is exempt from WCAG contrast, but don’t use this colour for anything readable.

#### Documentation site UX (OURS) · source: https://storybooks.fluentui.dev/react/
- The Storybook doubles as the docs: a left tree (Concepts, Theme, Components, Compat and Preview Components, Motion, Utilities) with live, editable stories and a props table under each.
- Live stories make state coverage easy to check; the trade-off is weaker narrative guidance than Carbon or Primer, which lives on fluent2.design.

#### Lessons to take (OURS) · source: https://storybooks.fluentui.dev/react/
- Ship themes as data so products can swap brands without touching components.
- Generate hover/pressed/selected from the brand ramp instead of hand-picking them.
- If your brand font can’t be shipped, design and test with the fallback that most users will actually see.

### Usage terms at a glance (our summary, not legal advice)
- Commercial use: Yes for code: Fluent UI React and @fluentui/react-theme are MIT. Fonts and icons referenced in Fluent UI React are governed by Microsoft’s assets licence (aka.ms/fluentui-assets-license); Segoe UI is not open, while the microsoft/fluentui-system-icons repo is MIT
- Attribution: Keep the MIT notice. Read the assets licence before bundling fonts or icons
- Redistribution: Code yes (MIT); Segoe UI no; Fluent UI System Icons MIT (check the licence of the package you install)
- How you get it: npm (@fluentui/react-components, @fluentui/web-components), Figma (Fluent 2 Web kit), GitHub

## Color tokens
OFFICIAL · Alias colour tokens (web light / web dark). OFFICIAL values from @fluentui/react-theme 9.2.2 (MIT, © Microsoft), objects webLightTheme and webDarkTheme, which Fluent UI React v9 documents in its public Storybook (Theme › Colors). fluent2.design itself refused TLS connections from our box on 2026-10-25.

| Role | Light | Dark |
|---|---|---|
| colorNeutralBackground1 · Default surface | `#ffffff` | `#292929` |
| colorNeutralBackground2 · Secondary surface | `#fafafa` | `#1f1f1f` |
| colorNeutralBackground3 · Tertiary surface | `#f5f5f5` | `#141414` |
| colorNeutralBackground1Hover · Surface hover | `#f5f5f5` | `#3d3d3d` |
| colorNeutralBackground1Pressed · Surface pressed | `#e0e0e0` | `#1f1f1f` |
| colorNeutralForeground1 · Primary text | `#242424` | `#ffffff` |
| colorNeutralForeground2 · Secondary text | `#424242` | `#d6d6d6` |
| colorNeutralForeground3 · Tertiary text | `#616161` | `#adadad` |
| colorNeutralForegroundDisabled · Disabled text | `#bdbdbd` | `#5c5c5c` |
| colorNeutralStroke1 · Default stroke | `#d1d1d1` | `#666666` |
| colorNeutralStroke2 · Subtle stroke | `#e0e0e0` | `#525252` |
| colorNeutralStrokeAccessible · Accessible stroke (inputs) | `#616161` | `#adadad` |
| colorBrandBackground · Primary (brand) button | `#0f6cbd` | `#115ea3` |
| colorBrandBackgroundHover · Brand hover | `#115ea3` | `#0f6cbd` |
| colorBrandBackgroundPressed · Brand pressed | `#0c3b5e` | `#0c3b5e` |
| colorBrandForeground1 · Brand text / selected | `#0f6cbd` | `#479ef5` |
| colorBrandForegroundLink · Links | `#115ea3` | `#479ef5` |
| colorNeutralForegroundOnBrand · Text on brand | `#ffffff` | `#ffffff` |
| colorStrokeFocus2 · Focus ring (outer) | `#000000` | `#ffffff` |
| colorPaletteRedForeground1 · Error text | `#bc2f32` | `#e37d80` |
| colorPaletteRedBackground3 · Error fill | `#d13438` | `#d13438` |
| colorPaletteGreenForeground1 · Success text | `#0e700e` | `#54b054` |
| colorPaletteGreenBackground3 · Success fill | `#107c10` | `#107c10` |
| colorPaletteYellowBackground3 · Warning fill | `#fde300` | `#fde300` |
| colorStatusWarningForeground1 · Warning text | `#bc4b09` | `#faa06b` |

ESTIMATED · dominant colors measured from our screenshot(s) of the source site (median-cut quantization; share = % of pixels). Approximate, not official:

- `#FFFFFF` ~67%
- `#EBD6F5` ~5%
- `#A3ACC5` ~5%
- `#CAB9DC` ~4%
- `#2E2E35` ~4%
- `#5A7BB3` ~3%

## Typography
OFFICIAL · Fluent type ramp (typographyStyles; Segoe UI, then -apple-system, BlinkMacSystemFont, Roboto, Helvetica Neue)

| Style | Weight | Size | Line height | Tracking |
|---|---|---|---|---|
| display | Semibold | 68 px (Hero1000) | 92 px | — |
| largeTitle | Semibold | 40 px (Hero900) | 52 px | — |
| title1 | Semibold | 32 px (Hero800) | 40 px | — |
| title2 | Semibold | 28 px (Hero700) | 36 px | — |
| title3 | Semibold | 24 px (Base600) | 32 px | — |
| subtitle1 | Semibold | 20 px (Base500) | 28 px | — |
| subtitle2 | Semibold | 16 px (Base400) | 22 px | — |
| body1 (default) | Regular | 14 px (Base300) | 20 px | — |
| body1Strong | Semibold | 14 px (Base300) | 20 px | — |
| body2 | Regular | 16 px (Base400) | 22 px | — |
| caption1 | Regular | 12 px (Base200) | 16 px | — |
| caption2 | Regular | 10 px (Base100) | 14 px | — |

## Spacing, radii, shadows
- OFFICIAL · Spacing (horizontal = vertical): None 0 · XXS 2 · XS 4 · SNudge 6 · S 8 · MNudge 10 · M 12 · L 16 · XL 20 · XXL 24 · XXXL 32 px
- OFFICIAL · Border radius: None 0 · Small 2 · Medium 4 · Large 6 · XLarge 8 · 2XL 12 · 3XL 16 · 4XL 24 · 5XL 32 · 6XL 40 px · Circular
- OFFICIAL · Stroke widths: Thin 1 · Thick 2 · Thicker 3 · Thickest 4 px
- OFFICIAL · Shadow ramp (light): shadow2: 0 0 2px rgba(0,0,0,0.12), 0 1px 2px rgba(0,0,0,0.14) · shadow4: 0 0 2px rgba(0,0,0,0.12), 0 2px 4px rgba(0,0,0,0.14) · shadow8: 0 0 2px rgba(0,0,0,0.12), 0 4px 8px rgba(0,0,0,0.14) · shadow16: 0 0 2px rgba(0,0,0,0.12), 0 8px 16px rgba(0,0,0,0.14) · shadow28: 0 0 8px rgba(0,0,0,0.12), 0 14px 28px rgba(0,0,0,0.14) · shadow64: 0 0 8px rgba(0,0,0,0.12), 0 32px 64px rgba(0,0,0,0.14)
- OFFICIAL · Durations: UltraFast 50 · Faster 100 · Fast 150 · Normal 200 · Gentle 250 · Slow 300 · Slower 400 · UltraSlow 500 ms
- OFFICIAL · Curves: EasyEase (0.33,0,0.67,1) · DecelerateMid (0,0,0,1) · AccelerateMid (1,0,1,1) · EasyEaseMax (0.8,0,0.2,1) · Linear
- OFFICIAL · Font families: Base: Segoe UI stack · Monospace: Consolas, Courier New · Numeric: Bahnschrift, Segoe UI
- OFFICIAL · Weights: Regular 400 · Medium 500 · Semibold 600 · Bold 700
- Gaps (not documented / not verified): Breakpoints and a column grid are not part of @fluentui/react-theme; Segoe UI is not openly licensed, so our specimens use Roboto, a fallback listed in Fluent’s own font stack.

## Components
- Button, CompoundButton, SplitButton, ToggleButton, MenuButton
- Accordion
- Avatar, AvatarGroup, Persona
- Badge, CounterBadge, PresenceBadge
- Card
- Checkbox, Radio, Switch
- Combobox, Dropdown, Select
- DataGrid, Table
- Dialog, Drawer
- Field, Input, Textarea, SpinButton
- Link
- Menu
- MessageBar, Toast
- Popover, Tooltip
- ProgressBar, Spinner, Skeleton
- Slider
- TabList
- Tag, TagPicker
- Toolbar
- Tree

## Layout rules
- Observed layout pattern: **Documentation** (from our captures).

## Usage
```
npm install @fluentui/react-components
import { FluentProvider, webLightTheme, Button, tokens } from '@fluentui/react-components';
<FluentProvider theme={webLightTheme}>
  <Button appearance="primary">Save</Button>
</FluentProvider>
// in makeStyles: { color: tokens.colorNeutralForeground1 }
```

## Do’s and don’ts
- Do: Keep chrome neutral and let content (images, icons, type) carry color.
- Don’t: Don’t introduce decorative color that competes with content.
- Do: follow the source’s license/terms listed above.
- Don’t: present ESTIMATED colors as official tokens.

## Related entries (ours)
- DS-002: Apple HIG is the other platform-owner system; both lean on a proprietary system font. · https://design-library-byt.pages.dev/entry/DS-002
- DS-003: Carbon is the flat, grid-strict counterpart to Fluent’s soft, shadowed surfaces. · https://design-library-byt.pages.dev/entry/DS-003
- DS-004: Primer’s light/dark functional tokens map closely to Fluent’s neutral alias tokens. · https://design-library-byt.pages.dev/entry/DS-004
- WR-004: Notion’s site shows a similar calm, neutral productivity aesthetic. · https://design-library-byt.pages.dev/entry/WR-004

## Sources
- https://fluent2.design/
- Library entry: https://design-library-byt.pages.dev/entry/DS-005