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CSS Modules are CSS files where all class names are locally scoped by default, meaning they're automatically transformed into unique identifiers at build time to prevent naming collisions. You import them into your JavaScript as an object and reference classes like `styles.button` instead of plain strings. This approach is natively supported by bundlers like Webpack, Vite, and Next.js, offering style isolation without the runtime overhead of CSS-in-JS.

JSX (JavaScript XML) is a syntax extension for JavaScript used primarily in React that lets you write HTML-like markup directly within your JavaScript code. It gets transpiled by tools like Babel or SWC into regular `React.createElement()` calls before reaching the browser. JSX supports embedding dynamic expressions with curly braces, conditional rendering, and mapping over arrays, making it a powerful and intuitive way to describe UI structures declaratively.

Headless UI refers to component libraries that provide behavior, state management, and accessibility logic without any predefined styling or markup. Libraries like Radix Primitives, Headless UI by Tailwind Labs, and React Aria give developers full control over visual presentation while handling complex patterns like focus management, keyboard navigation, and ARIA attributes. This approach decouples functionality from design, making it ideal for custom design systems.

CSS Custom Properties (also called CSS Variables) let you define reusable values with `--my-variable` syntax and reference them anywhere using `var(--my-variable)`. Unlike preprocessor variables in Sass or Less, they are live in the browser, cascade through the DOM, and can be updated dynamically with JavaScript. They're essential for theming systems, dark mode implementations, and maintaining consistent design tokens across large stylesheets.

Redux is a predictable state management library that stores the entire application state in a single immutable store, updated exclusively through dispatched actions and pure reducer functions. Redux Toolkit (RTK) is now the recommended way to use Redux, dramatically reducing boilerplate with utilities like `createSlice` and `createAsyncThunk`. While Redux remains popular in large enterprise applications that benefit from strict unidirectional data flow and time-travel debugging, lighter alternatives like Zustand have gained ground for simpler use cases.

Design tokens are the smallest units of a design system — named values representing colors, spacing, typography, border radii, and other visual properties. They act as a single source of truth that can be translated into platform-specific formats (CSS variables, Swift constants, XML resources). By changing token values in one place, you can update the look and feel of an entire application across multiple platforms simultaneously.