CSS Animation Reveal On Scroll Example

<!doctype html>
<title>Example</title>
<style>
body {
    margin: 0;
    font-family: sans-serif;
    background-color: #f0f0f0;
    min-height: 200vh; /* Make the page scrollable */
    display: flex;
    flex-direction: column;
    align-items: center;
}

.spacer {
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    text-align: center;
}

.reveal-card {
    background-color: #fff;
    padding: 30px;
    border-radius: 8px;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
    max-width: 400px;
    margin: 50px 20px;
    
    /* Starting state: hidden and pushed down */
    opacity: 0;
    transform: translateY(50px);
    transition: opacity 0.8s ease-out, transform 0.8s ease-out;
}

/* Final state when the JS adds this class */
.reveal-card.is-visible {
    opacity: 1;
    transform: translateY(0);
}

@media (prefers-reduced-motion: reduce) {
    .reveal-card {
        transition: none; /* Instant appearance */
        transform: none; /* Don't slide up, just fade or appear */
    }
}
</style>

<div class="spacer">
    <h1>Scroll down slowly &#x1F447;</h1>
</div>

<div class="reveal-card reveal-element">
    <h2>I magically appear!</h2>
    <p>This box faded in and slid up when it entered the viewport via the Intersection Observer API.</p>
</div>

<div class="reveal-card reveal-element" style="transition-delay: 0.2s;">
    <h2>I'm slightly delayed!</h2>
    <p>You can stagger animations simply by adding a transition-delay on adjacent items.</p>
</div>

<div class="spacer">
    <p>Keep scrolling...</p>
</div>

<div class="reveal-card reveal-element">
    <h2>Here comes another!</h2>
    <p>The observer keeps watching and firing precisely when elements cross the threshold.</p>
</div>

// Set up the Intersection Observer
const observer = new IntersectionObserver((entries, observer) => {
    entries.forEach(entry => {
        // If the element is visible in the viewport
        if (entry.isIntersecting) {
            entry.target.classList.add('is-visible');
            // Un-observe it if you only want the animation to happen once
            observer.unobserve(entry.target);
        }
    });
}, {
    root: null, // use the document's viewport
    rootMargin: '0px',
    threshold: 0.2 // trigger when 20% of the element is visible
});

// Start observing each element
revealElements.forEach(el => observer.observe(el));
</script>