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animation-on-scroll

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.

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SKILL.md
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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

<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

<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

<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?
Repository
MengTo/Skills
Path

agent-skills/web-design/animation-on-scroll/SKILL.md

Commit

46abf78

Last updated
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