Animation On Scroll

---
name: animation-on-scroll
description: Create an on-scroll animation trigger using IntersectionObserver
  with Tailwind-friendly animation classes and keyframes. Use when asked for
  scroll-reveal, animate-on-scroll, or sequencing element animations when they
  enter the viewport.
---

# Animation On Scroll Skill

## Workflow
1. Confirm animation style, timing, and whether animations should run once or repeat.
2. Provide the keyframes + JS observer snippet and the exact Tailwind class to apply.
3. Offer focused tweaks only (threshold, rootMargin, duration, delay, transform/blur values).

## Usage checklist
- Insert the JS snippet in the `<head>` after the keyframes.
- Add the animation class and `animate-on-scroll` to elements.
- Ensure your keyframes name matches the Tailwind animation reference.

## IntersectionObserver trigger
```html
<script>
  /*
    Sequence animation on scroll when visible. Requires Animation Keyframe. Usage:

    1) Insert this code in the <head> along with the Animation Keyframe code.

    2) Add to Tailwind Classes: [animation:animationIn_0.8s_ease-out_0.1s_both] animate-on-scroll
  */
  (function () {
    // Inject CSS for paused/running states
    const style = document.createElement("style");
    style.textContent = `
      /* Default: paused */
      .animate-on-scroll { animation-play-state: paused !important; }
      /* Activated by JS */
      .animate-on-scroll.animate { animation-play-state: running !important; }
    `;
    document.head.appendChild(style);

    const once = true;

    if (!window.__inViewIO) {
      window.__inViewIO = new IntersectionObserver((entries) => {
        entries.forEach((entry) => {
          if (entry.isIntersecting) {
            entry.target.classList.add("animate");
            if (once) window.__inViewIO.unobserve(entry.target);
          }
        });
      }, { threshold: 0.2, rootMargin: "0px 0px -10% 0px" });
    }

    window.initInViewAnimations = function (selector = ".animate-on-scroll") {
      document.querySelectorAll(selector).forEach((el) => {
        window.__inViewIO.observe(el); // observing twice is a no-op
      });
    };

    document.addEventListener("DOMContentLoaded", () => initInViewAnimations());
  })();
</script>
```

## Keyframes
```html
<style>
  /*
    Sequence animation intro. Usage:

    1) Insert this code in the <head>

    2) Add to Tailwind Classes: [animation:animationIn_0.8s_ease-out_0.1s_both]
  */
  @keyframes animationIn {
    0% {
      opacity: 0;
      transform: translateY(30px);
      filter: blur(8px);
    }

    100% {
      opacity: 1;
      transform: translateY(0);
      filter: blur(0px);
    }
  }
</style>
```

## Tailwind example
```html
<div class="animate-on-scroll [animation:animationIn_0.8s_ease-out_0.1s_both]">
  ...
</div>
```

## Customization knobs
- Trigger: adjust `threshold` and `rootMargin` for earlier/later reveals.
- Repeat: set `once = false` to allow replays when re-entering.
- Motion: tweak `translateY` and `blur` in keyframes.
- Timing: change duration and delay in the Tailwind animation value.

## Common pitfalls
- Forgetting to include the keyframes before the JS snippet.
- Using a different keyframe name than in the Tailwind animation.
- Animations not running because the element is already in view before observer init.

## Questions to ask when specs are missing
- Should animations run once or every time the element re-enters?
- How far before entering the viewport should they start?
- What motion style (fade, slide, blur, scale) do you want?