SKILL

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.

● INSTALL

This skill ships inside the skills package. Install the package to get this skill plus everything else in the bundle.

sv install mengto/skills

● SKILL.MD

Animation On Scroll Skill

Workflow

Confirm animation style, timing, and whether animations should run once or repeat.
Provide the keyframes + JS observer snippet and the exact Tailwind class to apply.
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?