CSS Animation Glassmorphism Shimmer 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: #1a1a2e;
    background-image: radial-gradient(circle at 15% 50%, rgba(20, 30, 70, 0.4), transparent 25%), radial-gradient(circle at 85% 30%, rgba(30, 10, 50, 0.4), transparent 25%);
}

.glass-card {
    position: relative;
    width: 300px;
    height: 400px;
    background: rgba(255, 255, 255, 0.05);
    border-radius: 15px;
    border: 1px solid rgba(255, 255, 255, 0.1);
    backdrop-filter: blur(10px);
    -webkit-backdrop-filter: blur(10px);
    overflow: hidden;
    color: white;
    display: flex;
    flex-direction: column;
    justify-content: center;
    align-items: center;
    box-shadow: 0 8px 32px 0 rgba(0, 0, 0, 0.37);
}

/* The Shimmer Effect */
.glass-card::before {
    content: '';
    position: absolute;
    top: 0;
    left: -100%; /* Start entirely off-card to the left */
    width: 50%;
    height: 100%;
    /* A skewed, semi-transparent white gradient passing over */
    background: linear-gradient(
        to right,
        rgba(255, 255, 255, 0) 0%,
        rgba(255, 255, 255, 0.2) 50%,
        rgba(255, 255, 255, 0) 100%
    );
    transform: skewX(-20deg);
    animation: shimmer 3s infinite linear;
}

@keyframes shimmer {
    100% {
        left: 200%; /* Move completely across and off-card to the right */
    }
}

@media (prefers-reduced-motion: reduce) {
    .glass-card::before {
        display: none; /* Completely removes the moving shimmer */
    }
}
</style>

<div class="glass-card">
    <h2>Pro Plan</h2>
    <p>$19.99 / mo</p>
    <button style="padding: 10px 20px; border-radius: 20px; border: none; font-weight: bold; margin-top: 20px; cursor: pointer;">Upgrade</button>
</div>