CSS Animation Generator — Keyframe & Animation Builder

The @keyframes rule defines animation frames by specifying CSS property values at percentage points along the animation timeline. The from keyword is equivalent to 0% (start state) and to is equivalent to 100% (end state). You can add intermediate frames: @keyframes slide { 0% { opacity: 0; transform: translateY(-20px); } 60% { opacity: 1; transform: translateY(5px); } 100% { transform: translateY(0); } }. Each percentage point is a keyframe where the browser interpolates property values between them. The browser automatically calculates the intermediate states using the specified timing function, creating smooth transitions between keyframes.

The animation shorthand value order is: name duration timing-function delay iteration-count direction fill-mode play-state. Example: animation: fadeIn 0.4s ease-out 0s 1 normal both running. The most important properties are: duration (how long one cycle takes), timing-function (easing curve — ease, linear, ease-in, ease-out, ease-in-out, cubic-bezier()), iteration-count (number of repetitions, or infinite), and fill-mode (what styles apply before and after the animation).

animation-fill-mode controls what styles apply outside the active animation period. none (default): the element reverts to its original CSS styles before and after the animation — the animated values only apply during the duration. forwards: after the animation ends, the element retains the styles from the final keyframe (100%). This is what makes elements "stay" in their animated state. backwards: before the animation starts (during delay), the from (0%) keyframe styles apply rather than the element's original styles. both: combines forwards and backwards — the most useful setting for entrance animations and one-time effects.

Modern browsers can offload certain CSS animations to the GPU, running them on a separate compositor thread independent of JavaScript execution. The properties that trigger GPU acceleration are: transform (all functions: translate, scale, rotate, skew) and opacity. These two properties can animate at 60fps even when the main thread is busy. All other CSS properties — width, height, top, left, background-color, border-radius, box-shadow — cause layout recalculation or paint operations on the main thread, which is much more expensive. For performance-critical animations like loaders, transitions, and scroll effects, restrict your animations to transform and opacity whenever possible.

The will-change property tells the browser in advance that an element will be animated, allowing it to optimize rendering by promoting the element to its own compositor layer. Example: will-change: transform, opacity. This creates a new stacking context and allocates GPU memory for the element before the animation starts, eliminating the visual jank sometimes seen on the first frame of complex animations. Use will-change sparingly — applying it to many elements simultaneously consumes significant GPU memory and can degrade performance. Apply it only to elements you are actively about to animate, and remove it after the animation completes if possible.

Some users experience motion sickness, dizziness, or cognitive overload from animations, particularly those involving large movements, rapid flashing, or parallax effects. The prefers-reduced-motion media query lets you disable or simplify animations for these users: @media (prefers-reduced-motion: reduce) { .animated-element { animation: none; transition: none; } }. Implementing this is a WCAG 2.1 AAA criterion and is considered good accessibility practice for any animation-heavy interface. Users can enable this preference in macOS (System Settings → Accessibility → Display → Reduce Motion) and Windows (Settings → Ease of Access → Display → Show animations).