CSS Animation Infinite Marquee Example

<!doctype html>
<title>Example</title>
<style>
body {
    display: flex;
    justify-content: center;
    align-items: center;
    height: 100vh;
    margin: 0;
    font-family: sans-serif;
    background-color: #f0f0f0;
}

/* The window that crops the overflow */
.marquee-container {
    width: 600px;
    background-color: white;
    padding: 20px 0;
    overflow: hidden; /* Crucial */
    position: relative;
    box-shadow: 0 4px 6px rgba(0,0,0,0.1);
    
    /* Adds a slight fade to the edges using a CSS mask */
    -webkit-mask-image: linear-gradient(
        to right, 
        transparent, 
        black 10%, 
        black 90%, 
        transparent
    );
    mask-image: linear-gradient(
        to right, 
        transparent, 
        black 10%, 
        black 90%, 
        transparent
    );
}

/* 
   The track that holds all the items. Note it contains TWO sets of identical items.
   We use flex-shrink: 0 so the track doesn't naturally crush its children to fit.
*/
.marquee-track {
    display: flex;
    gap: 30px; /* Space between logos */
    width: max-content;
    /* 
       To calculate the animation: 
       We animate the track exactly 50% of its width.
       Because the track contains exactly two copies of the same items, 
       shifting it left by 50% perfectly aligns copy B onto the starting spot of copy A.
    */
    animation: scroll-left 10s linear infinite;
}

/* Hovering the container stops the track for easier reading/clicking */
.marquee-container:hover .marquee-track {
    animation-play-state: paused;
}

.marquee-item {
    background-color: #007bff;
    color: white;
    padding: 10px 20px;
    border-radius: 4px;
    font-weight: bold;
    flex-shrink: 0;
}

@keyframes scroll-left {
    from {
        transform: translateX(0);
    }
    to {
        transform: translateX(calc(-50% - 15px)); 
        /* The -15px is exactly half of the gap space to make the math seamless */
    }
}

@media (prefers-reduced-motion: reduce) {
    .marquee-track {
        animation: none; /* Stops the scrolling */
        flex-wrap: wrap; /* Allows them to stack instead of hiding off-screen */
        width: 100%;
        gap: 10px;
        justify-content: center;
    }
    .marquee-container {
        /* Removes the mask so the wrapped items are visible on the edges */
        mask-image: none;
        -webkit-mask-image: none;
    }
    .marquee-item[aria-hidden="true"] {
        display: none; /* Hide duplicates */
    }
}
</style>