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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.

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.

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.

Progressive enhancement is a web development strategy that starts with a baseline of functional HTML content accessible to all browsers, then layers on CSS styling and JavaScript interactivity for more capable environments. This approach ensures that core functionality works everywhere, while users with modern browsers get a richer experience. It contrasts with graceful degradation, which starts with the full experience and tries to handle failures.

Lazy loading is a performance optimization technique that defers loading of non-critical resources until they are actually needed — typically when they enter the viewport. It is commonly applied to images, videos, and below-the-fold components to speed up initial page load. Modern browsers support native lazy loading via the `loading="lazy"` attribute, while JavaScript-based solutions offer more control through Intersection Observer.

Focus management is the practice of controlling which element receives keyboard focus in a web application, especially during dynamic content changes. When a modal opens, focus should move into it; when it closes, focus should return to the trigger element. Proper focus management is essential for keyboard and screen reader users, and involves techniques like focus trapping, roving tabindex, and programmatic focus with `element.focus()`.