Progressive Enhancement

Code splitting is an optimization technique that breaks a JavaScript bundle into smaller chunks that are loaded on demand, rather than forcing users to download the entire application upfront. In React, this is achieved through `React.lazy()` and dynamic `import()` statements, which create separate bundles loaded only when the corresponding component or route is accessed. This dramatically improves initial load times, especially for large single-page applications with many routes.

Atomic Design is a methodology by Brad Frost for creating design systems by breaking interfaces into five hierarchical levels: atoms (basic HTML elements), molecules (simple component groups), organisms (complex UI sections), templates (page-level layouts), and pages (specific instances with real content). This approach provides a consistent mental model for organizing component libraries in React, React Native, or Flutter projects and ensures systematic scalability of the UI.

Cross-platform development is the practice of building applications that run on multiple operating systems (iOS, Android, web, desktop) from a single codebase. Frameworks like React Native, Flutter, and Kotlin Multiplatform each take different approaches — from shared UI rendering to shared business logic with native UI. The key trade-off is between code reuse efficiency and the ability to deliver platform-native experiences that feel right on each device.

In Flutter, the widget tree is the hierarchical structure of nested widgets that defines the entire UI of an application. Everything in Flutter is a widget — from layout structures like `Column` and `Row` to styling wrappers like `Padding` and `Theme` to interactive elements like `GestureDetector`. Flutter maintains three parallel trees (widget, element, and render object) that work together to efficiently rebuild only the parts of the UI that actually changed when state updates.

A wireframe is a low-fidelity visual representation of a user interface that outlines structure, layout, and content hierarchy without detailed styling or colors. Wireframes focus on what elements appear on a page and how they are arranged, serving as a blueprint before visual design begins. They help teams align on functionality and user flow early in the process, when changes are cheapest to make.

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.