b2KIT

Scroll Animation Builder

Design scroll-triggered CSS animations with visual keyframe editor and IntersectionObserver code output.

Tested tool guide Tested browser tools Checked August 16, 2026

What Scroll Animation Builder does and how it behaves

You compose the motion visually: give an element a start state, an end state, a duration, an easing, and the viewport position where it should play, and the tool turns that into CSS plus a small JavaScript snippet that starts the animation when the element scrolls into view. The motion is visibility-triggered rather than scroll-position-triggered, and that is what surprises people first: anything already on screen when the page loads fires immediately, so above-the-fold content finishes its entrance before anyone scrolls. If the reveal is meant to appear on arrival, you are tuning the trigger point, not the scroll position.

How the result is produced

1

Visual keyframe editor to CSS

The editor exposes each animated property as a control across two states, usually a hidden start (opacity 0, a translate or scale offset) and the visible end. From those values it derives plain CSS: a base rule, a visible-state class or @keyframes, plus the duration, delay, and easing curve you picked. The result is plain CSS you can paste into a stylesheet as-is.

2

IntersectionObserver trigger

The generated JavaScript creates an IntersectionObserver for the element. The threshold you set is the fraction of the element's own box that must be visible before the callback fires - 0.2 means about one fifth of the element, not 20 pixels and not 20% of the viewport. A rootMargin you set expands or shrinks the viewport rectangle used for the check. The callback flips a class, which starts the CSS animation.

Good uses

  • Reveal effects on a landing page: fade and slide feature cards, section headings, and gallery rows in as they reach the viewport, with one consistent treatment across the page.
  • Entrance motion on a long article or marketing site without hand-writing an observer per element - define the effect once and reuse it for many targets.
  • Trying out duration, easing, and trigger point for an element before committing to code, since the visual timeline shows the result immediately.

Limits and checks

  • Elements already visible at page load animate immediately, because the observer reports current visibility on startup. Above-the-fold content plays its entrance once, at load, rather than when the user scrolls to it.
  • The threshold is a ratio of the element's own size, not pixels and not scroll distance. A tall element crosses 0.2 sooner in its journey across the screen than a short one, and an element taller than the viewport may never reach thresholds near 1.
  • If the hidden start state ships in the page's static CSS, content at opacity 0 stays invisible whenever the script cannot run - blocked, errored, or disabled. Check whether the generated snippet applies the hidden class only after the script starts; that decides whether the page degrades gracefully.

Common questions

Why has my animation already finished when I scroll to the element?

The observer's first callback reports the element's current visibility the moment observation starts, so anything inside the viewport at page load triggers instantly - the entrance plays before any scrolling. This is standard IntersectionObserver behavior, not a tool bug. Raising the threshold or setting a negative rootMargin delays the trigger, but visible-at-load elements still fire that initial callback, so the motion plays at load unless the snippet ignores it.

Can the animation replay when I scroll back up to the element?

Only if the callback also handles the element leaving: it must remove the visible class when the element exits the viewport, so re-entering plays the animation again. A snippet that merely adds the class on entry plays once per page load. Check the copied code for an isIntersecting false branch; if it is missing, add one, or accept the one-shot behavior.

References and verification

The behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.

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