CSS Accordion - Collapsible Menu Element

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Accordion Menu Component</title>
    <meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1">
    <link rel="stylesheet" href="style.css">
    <script src="script.js" defer></script>
</head>
<body>
    <div class="accordion-container">
        <button class="accordion-trigger">Section 1</button>
        <div class="accordion-panel">
            <h3>Welcome to Section 1</h3>
            <p>This is the content of the first section inside the accordion menu. Everything is fast and responsive.</p>
        </div>

        <button class="accordion-trigger">Section 2</button>
        <div class="accordion-panel">
            <h3>Explore Section 2</h3>
            <p>Here you can find the content of the second section with extra details.</p>
        </div>

        <button class="accordion-trigger">Section 3</button>
        <div class="accordion-panel">
            <h3>Section 3 Title</h3>
            <p>Content of the third section providing additional information for the end user.</p>
        </div>
    </div>
</body>
</html>

:root {
    --acc-bg: var(--bg, #f8fafc);
    --acc-card: var(--card, #ffffff);
    --acc-fg: var(--fg, #1e293b);
    --acc-border: var(--border, #e2e8f0);
    --acc-accent: var(--accent, #007bff);
}

.accordion-container {
    max-width: 650px;
    margin: 2rem auto;
    padding: 0 15px;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
}

.accordion-trigger {
    width: 100%;
    padding: 16px 20px;
    background: var(--acc-card);
    color: var(--acc-fg);
    border: 1px solid var(--acc-border);
    border-radius: 8px;
    margin-bottom: 8px;
    cursor: pointer;
    font-size: 1rem;
    font-weight: 600;
    text-align: left;
    display: flex;
    justify-content: space-between;
    align-items: center;
    transition: background-color 0.2s ease, border-color 0.2s ease, color 0.2s ease;
    outline: none;
}

/* Dynamic plus (+) indicator on the right side of the trigger button */
.accordion-trigger::after {
    content: "+";
    font-size: 1.2rem;
    font-weight: 400;
    color: #94a3b8;
    transition: transform 0.25s ease;
}

.accordion-trigger:hover {
    background: rgba(0, 123, 255, 0.04);
    border-color: var(--acc-accent);
    color: var(--acc-accent);
}

/* Active state of the button when open */
.accordion-trigger.is-active {
    background: var(--acc-accent);
    color: #ffffff !important;
    border-color: var(--acc-accent);
    border-bottom-left-radius: 0;
    border-bottom-right-radius: 0;
    margin-bottom: 0; /* Merges the button with the panel beneath */
}

/* Changes the plus sign into a minus sign (−) upon activation */
.accordion-trigger.is-active::after {
    content: "−";
    color: #ffffff;
}

/* Accordion content area wrapper */
.accordion-panel {
    display: none;
    padding: 24px 20px;
    background: var(--acc-card);
    color: var(--acc-fg);
    border: 1px solid var(--acc-border);
    border-top: none;
    border-bottom-left-radius: 8px;
    border-bottom-right-radius: 8px;
    margin-bottom: 12px;
}

.accordion-panel.is-visible {
    display: block;
    animation: accordionFadeIn 0.3s cubic-bezier(0.16, 1, 0.3, 1) forwards;
}

@keyframes accordionFadeIn {
    from { opacity: 0; transform: translateY(-4px); }
    to { opacity: 1; transform: translateY(0); }
}

.accordion-panel h3 {
    font-size: 1.15rem;
    font-weight: 700;
    margin-bottom: 10px;
    letter-spacing: -0.01em;
}

.accordion-panel p {
    font-size: 0.95rem;
    line-height: 1.6;
    opacity: 0.85;
}

@media (max-width: 480px) {
    .accordion-container { margin: 1rem auto; }
    .accordion-trigger { padding: 14px 16px; font-size: 0.95rem; }
    .accordion-panel { padding: 20px 16px; }
}

// Safely wait until the full HTML DOM tree is loaded into the iframe simulator
document.addEventListener("DOMContentLoaded", () => {
    const accordionButtons = document.querySelectorAll(".accordion-trigger");
            
    accordionButtons.forEach(btn => {
        btn.addEventListener("click", function() {
            const panel = this.nextElementSibling;
            if (!panel) return;

            const isActive = this.classList.contains("is-active");
            
            // Close all other open sections (Classic Accordion behavior)
            accordionButtons.forEach(otherBtn => {
                otherBtn.classList.remove("is-active");
                if (otherBtn.nextElementSibling) {
                    otherBtn.nextElementSibling.classList.remove("is-visible");
                }
            });
            
            // Open the current section only if it was not already active
            if (!isActive) {
                this.classList.add("is-active");
                panel.classList.add("is-visible");
            }
        });
    });
});