Implement Apple HIG Design Foundations

Compiles Apple's Human Interface Guidelines design foundations - the cross-cutting rules that apply to every screen and platform - into eight synthesized Key Principles plus full Apple reference docs for eighteen topics: accessibility, app icons, branding, color, dark mode, icons, images, immersive experiences, inclusion, layout, materials, motion, privacy, right-to-left, SF Symbols, spatial layout, typography, and writing. The principles set the baseline: prioritize content over chrome (system materials and subtle separators over heavy borders), build in accessibility from day one (VoiceOver, Dynamic Type, Reduce Motion, Increase Contrast, Switch Control, with an accessible label on every interactive element), prefer system colors and semantic tokens - label, secondaryLabel, systemBackground - over hard-coded values so they adapt across light/dark mode and vibrancy, use platform fonts (SF Pro, SF Compact, SF Mono, New York for serif) and SF Symbols for iconography, match platform conventions for look, behavior, and feedback, respect privacy by requesting permissions only when needed and explaining why, support internationalization (text expansion, right-to-left scripts, locale-specific date and number formats via Auto Layout), and use motion purposefully with a Reduce Motion crossfade alternative for every animation. A five-item "Applying Foundations Together" section calls out where principles interact and must be checked jointly - for example, custom color palettes must hold WCAG contrast ratios in both light and dark mode, and Dynamic Type must scale without breaking layouts.

Use it as the baseline reference whenever making a foundational design decision - color palette, typography scale, iconography, motion, privacy-permission wording, or internationalization - on any Apple platform, especially before layering platform- or pattern-specific guidance on top. It complements rather than replaces three sibling HIG skills: hig-platforms covers how these foundations differ per platform (type-scale differences between watchOS and macOS, for example), hig-patterns covers interaction patterns that lean on these foundations, and hig-components-layout/hig-components-content cover the structural and content components that implement them. It is not a component library or code generator - the Output Format calls for citing the specific HIG foundation with file and section, noting platform differences, and only then giving a concrete SwiftUI/UIKit/AppKit code pattern.

There is no API - output is a cited foundation reference (file and section), platform-specific notes for the target platforms, a concrete code pattern, and an explicit accessibility-impact statement covering contrast ratios, Dynamic Type scaling, and VoiceOver behavior. Before answering, the skill asks which platforms are targeted, whether existing brand guidelines apply, what accessibility bar is the target (WCAG AA, AAA, or Apple's own baseline), and whether system or custom colors are in play.

References Apple's own design and accessibility APIs by name where its docs specify them - semantic color tokens like label, secondaryLabel, and systemBackground, SF Pro/SF Compact/SF Mono/New York fonts, and SF Symbols - and is meant to be read alongside three sibling HIG skills (hig-platforms, hig-patterns, hig-components-layout) rather than standalone, since foundational rules like color and typography only become actionable once applied to a specific platform or component.

Designers and developers making foundational design decisions - color, typography, iconography, motion, accessibility, privacy copy, or internationalization - for a native Apple-platform app, who want Apple's own baseline rules and cross-cutting interactions, such as color plus dark mode plus accessibility, rather than researching each of the eighteen underlying topics from Apple's documentation site individually.