Creating complex animations with CSS and JavaScript requires a solid understanding of how the browser renders frames and how to write performant, expressive code. The starting point is knowing that every animation must fit within a 16ms window to maintain a stable 60fps. This means costly layout operations or heavy DOM manipulations can easily break smoothness. The key is to rely on properties that don’t trigger reflow, such as `transform` and `opacity`, allowing the GPU to handle most of the work. With pure CSS, it’s possible to achieve impressive results, especially when working with keyframes and transitions. The limitation comes when animation logic needs to be dynamic, reacting to real-time data or complex user interactions. That’s where JavaScript naturally extends the possibilities. The `requestAnimationFrame` API is fundamental for syncing animations with the rendering cycle, avoiding the common mistake of using `setInterval`, which causes jank because it doesn’t respect the screen’s refresh rate. A classic example is direct manipulation of transforms in an optimized loop: ```javascript function animate(element, start, end, duration) { const startTime = performance.now(); function step(currentTime) { const progress = Math.min((currentTime - startTime) / duration, 1); const eased = 1 - Math.pow(1 - progress, 3); // easeOutCubic const value = start + (end - start) * eased; element.style.transform = `translateX(${value}px)`; if (progress < 1) requestAnimationFrame(step); } requestAnimationFrame(step); } ``` This approach gives full control over easing, synchronization, and the composition of multiple elements—something difficult to achieve with CSS alone. Interestingly, many libraries like **GSAP** or **Framer Motion** build on this same principle, offering abstractions that make it easier to orchestrate sequences and complex timelines. Another essential concept is **animation composition**. Isolated animations are simple, but the challenge comes when multiple animations must run in sync—for example, a card rotating while its content fades out and an internal chart redraws itself. Timelines solve this problem by letting you chain animations like a musical score, each with its own entry, exit, and rhythm. Performance is another cornerstone. Animating properties like `width`, `height`, or `top` is a common mistake because those trigger repaint and reflow. The right approach is to prioritize `transform` (for movement, rotation, scaling) and `opacity`. This ensures the GPU handles the work on the compositor thread without blocking layout calculations. Monitoring with tools like **Chrome DevTools Performance Panel** is indispensable for identifying hidden bottlenecks. In the end, mastering complex animations means combining **motion design** (the artistic side) with **rendering engineering** (the technical side). CSS offers declarative fluidity for static cases, while JavaScript delivers precision and reactivity for dynamic scenarios. When these two worlds meet, the result is visual experiences that not only beautify interfaces but also guide user attention and communicate states almost intuitively.