Internationalization

Accessibility (often abbreviated a11y) is the practice of designing and building websites and apps so they can be used by everyone, including people with visual, motor, auditory, or cognitive disabilities. This involves proper semantic markup, keyboard navigation support, sufficient color contrast, and screen reader compatibility. Beyond being an ethical responsibility, accessibility is increasingly a legal requirement in many jurisdictions.

Tree shaking is a dead code elimination technique used by modern bundlers like Webpack, Rollup, and esbuild to remove unused exports from the final JavaScript bundle. It relies on the static structure of ES module `import`/`export` syntax to determine which code is actually referenced and safely discard the rest. For tree shaking to work effectively, libraries must use ESM format and avoid side effects in their module initialization, which is why the `sideEffects` field in `package.json` matters.

A design system is a collection of reusable components, guidelines, and standards that ensure visual and functional consistency across a product or suite of products. It typically includes a component library, design tokens, typography rules, color palettes, and documentation. Design systems bridge the gap between designers and developers, enabling teams to build interfaces faster while maintaining a unified look and feel.

A component library is a curated set of pre-built, reusable UI elements — buttons, inputs, modals, cards — packaged for consistent use across projects. Libraries like Material UI, Chakra UI, or Radix provide accessible, themeable building blocks so developers don't have to reinvent common patterns. A well-maintained component library drastically reduces development time and ensures visual consistency throughout an application.

CSS Animation uses `@keyframes` rules and the `animation` property to create complex, multi-step animations entirely in CSS without JavaScript. Combined with CSS transitions for simple state changes, these tools handle most UI animation needs performantly since browsers can optimize them on the GPU compositor thread. Properties like `transform` and `opacity` are particularly efficient to animate because they don't trigger layout recalculations or repaints.

Dark mode is a UI color scheme that uses light-colored text and elements on a dark background. It reduces eye strain in low-light environments, can save battery on OLED screens, and has become an expected feature in modern applications. Implementation typically involves CSS custom properties or design tokens that switch between light and dark theme values, often respecting the user's system-level preference via the `prefers-color-scheme` media query.