Horizontal Tabs with CSS and JavaScript

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Horizontal Tabs - Example</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="horizontal-tabs-container">
        <div class="tab-header">
            <button class="links" onclick="openTab(event, 'tab1')">Tab 1</button>
            <button class="links" onclick="openTab(event, 'tab2')">Tab 2</button>
            <button class="links" onclick="openTab(event, 'tab3')">Tab 3</button>
        </div>
        
        <div class="content-box">
            <div id="tab1" class="content">
                <h2>First Content</h2>
                <p>This is the content of the first tab with a smooth and soft transition animation.</p>
            </div>
            
            <div id="tab2" class="content">
                <h2>Second Content</h2>
                <p>Here you can display completely different data, forms, or images.</p>
            </div>
            
            <div id="tab3" class="content">
                <h2>Third Content</h2>
                <p>Customizing the number of tabs is fast, easy, and clear.</p>
            </div>
        </div>
    </div>
</body>
</html>

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

.horizontal-tabs-container {
    width: 100%;
    max-width: 100%;
    padding: 20px 15px;
    background-color: var(--bg, #f8fafc);
    box-sizing: border-box;
}

/* Isolated elements reset inside the component example */
.horizontal-tabs-container *,
.horizontal-tabs-container *::before,
.horizontal-tabs-container *::after {
    box-sizing: border-box;
    font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, sans-serif;
    margin: 0;
    padding: 0;
}

/* Tab Header */
.horizontal-tabs-container .tab-header {
    display: flex;
    background-color: var(--tab-card-bg);
    border: 1px solid var(--tab-border);
    border-bottom: none;
    border-radius: 12px 12px 0 0;
    padding: 8px 12px 0 12px;
    gap: 4px;
}

.horizontal-tabs-container .links {
    padding: 12px 24px;
    cursor: pointer;
    border: none;
    background: transparent;
    font-size: 0.95rem;
    font-weight: 500;
    color: var(--tab-text);
    border-radius: 8px 8px 0 0;
    transition: all 0.2s ease;
    position: relative;
    outline: none;
}

.horizontal-tabs-container .links:hover {
    background-color: rgba(0, 123, 255, 0.04);
    color: var(--tab-accent);
}

/* Modern style for the active tab with an underline */
.horizontal-tabs-container .links.active {
    color: var(--tab-accent);
    font-weight: 600;
}

/* Elegant active bottom underline */
.horizontal-tabs-container .links::after {
    content: "";
    position: absolute;
    bottom: 0;
    left: 0;
    right: 0;
    height: 3px;
    background-color: var(--tab-accent);
    transform: scaleX(0);
    transition: transform 0.2s ease;
    border-radius: 3px 3px 0 0;
}

.horizontal-tabs-container .links.active::after {
    transform: scaleX(1);
}

/* Content box wrapper under the tabs */
.horizontal-tabs-container .content-box {
    background: var(--tab-card-bg);
    padding: 35px;
    border-radius: 0 0 12px 12px;
    border: 1px solid var(--tab-border);
    box-shadow: 0 10px 25px -5px rgba(0, 0, 0, 0.05);
    min-height: 180px;
}

.horizontal-tabs-container .content {
    display: none;
}

/* Soft and smooth content slide-up transition animation */
@keyframes tabSlideUp {
    from { opacity: 0; transform: translateY(6px); }
    to { opacity: 1; transform: translateY(0); }
}

.horizontal-tabs-container .content.active {
    display: block;
    animation: tabSlideUp 0.3s cubic-bezier(0.16, 1, 0.3, 1) forwards;
}

.horizontal-tabs-container .content h2 {
    margin-bottom: 12px;
    font-size: 1.35rem;
    font-weight: 700;
    color: var(--tab-text);
    letter-spacing: -0.02em;
}

.horizontal-tabs-container .content p {
    line-height: 1.65;
    color: var(--tab-text);
    opacity: 0.85;
    font-size: 0.98rem;
}

/* Mobile responsiveness (under 480px) */
@media (max-width: 480px) {
    .horizontal-tabs-container .tab-header {
        flex-direction: column;
        padding: 10px;
        border-radius: 12px;
        border-bottom: 1px solid var(--tab-border);
    }
    
    .horizontal-tabs-container .links {
        border-radius: 6px;
        text-align: center;
        width: 100%;
        padding: 10px;
    }

    .horizontal-tabs-container .links::after {
        display: none; /* Disable the horizontal bottom line on mobile devices */
    }

    /* Instead of an underline, we use a nice colored background on mobile devices */
    .horizontal-tabs-container .links.active {
        background-color: var(--tab-accent);
        color: #ffffff !important;
    }
    
    .horizontal-tabs-container .content-box {
        margin-top: 15px;
        border-radius: 12px;
        padding: 25px 20px;
    }
}

// 1. CRUCIAL: We expose the openTab function to the "window" object so it works via inline HTML onclick events!
window.openTab = function(evt, tabId) {
    const contents = document.querySelectorAll(".horizontal-tabs-container .content");
    const links = document.querySelectorAll(".horizontal-tabs-container .links");

    if (contents.length === 0 || links.length === 0) return;

    contents.forEach(content => content.classList.remove("active"));
    links.forEach(link => link.classList.remove("active"));

    const targetContent = document.getElementById(tabId);
    if (targetContent) {
        targetContent.classList.add("active");
    }
    
    evt.currentTarget.classList.add("active");
}

// Automatically click the first tab when the DOM content has loaded
document.addEventListener("DOMContentLoaded", () => {
    const firstTab = document.querySelector(".horizontal-tabs-container .links");
    if (firstTab && !firstTab.classList.contains("active")) {
        firstTab.click();
    }
});