Responsive Vertical Tabs Layout with CSS and JavaScript

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Vertical 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="vertical-tabs-container">
        <div class="zavihek">
            <span class="links active" onclick="openSub(event, 'zavihek1')">Home</span>
            <span class="links" onclick="openSub(event, 'zavihek2')">About Me</span>
            <span class="links" onclick="openSub(event, 'zavihek3')">Services</span>
            <span class="links" onclick="openSub(event, 'zavihek4')">Projects</span>
            <span class="links" onclick="openSub(event, 'zavihek5')">Contact</span>
        </div>
    
        <div id="zavihek1" class="content">
            <h2>Welcome to My Site</h2>
            <p>I am a passionate programmer who loves exploring the world of web development. Here I share my knowledge and experiences.</p>
        </div>
        <div id="zavihek2" class="content hidden">
            <h2>About Me</h2>
            <p>I am a developer who learns something new every day. My programming journey is full of challenges and discoveries.</p>
        </div>
        <div id="zavihek3" class="content hidden">
            <h2>My Work</h2>
            <p>I focus on developing websites and applications where I can apply my knowledge of HTML, CSS, PHP, and JavaScript.</p>
        </div>
        <div id="zavihek4" class="content hidden">
            <h2>Projects</h2>
            <p>Here you can view my projects - each of them represents a new step in my professional growth.</p>
        </div>
        <div id="zavihek5" class="content hidden">
            <h2>Contact</h2>
            <p>I am always open to new connections and collaborations. I am happy to share my experiences with you!</p>
        </div>
    </div>
</body>
</html>

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

.vertical-tabs-container {
    width: 100%;
    max-width: 100%; 
    display: flex;
    min-height: 400px;
    margin: 2rem auto;
    border-radius: 12px;
    border: 1px solid var(--border-color);
    box-shadow: 0 4px 20px rgba(0, 0, 0, 0.03);
    background-color: var(--card-bg);
    box-sizing: border-box; 
    overflow: hidden;
}

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

/* Sidebar wrapper for buttons */
.vertical-tabs-container .zavihek {
    width: 220px; 
    flex-shrink: 0; 
    background: var(--bg, #f8fafc);
    border-right: 1px solid var(--border-color);
    padding: 12px 8px;
    display: flex;
    flex-direction: column;
    gap: 4px;
}

.vertical-tabs-container .links {
    display: block;
    padding: 12px 18px;
    font-size: 0.95rem;
    font-weight: 500;
    color: var(--text-main);
    cursor: pointer;
    transition: all 0.2s ease;
    border-radius: 8px;
}

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

/* Style for the currently active/selected tab */
.vertical-tabs-container .links.active {
    background: var(--accent-color);
    color: #ffffff !important;
    box-shadow: 0 4px 10px rgba(0, 123, 255, 0.15);
}

/* Right-side content area */
.vertical-tabs-container .content {
    flex: 1;
    min-width: 0;
    padding: 35px;
    overflow-y: auto;
    background: var(--card-bg);
    color: var(--text-main);
}

.vertical-tabs-container .content.hidden {
    display: none;
}

.vertical-tabs-container .content h2 {
    margin-bottom: 14px;
    font-size: 1.4rem;
    font-weight: 700;
    letter-spacing: -0.02em;
}

.vertical-tabs-container .content p {
    color: var(--text-main);
    opacity: 0.85;
    line-height: 1.65;
    font-size: 0.98rem;
    word-wrap: break-word;
}

/* Smooth and soft content fade-in transition animation */
@keyframes tabFadeIn {
    from {
        opacity: 0; 
        transform: scale(0.99) translateY(4px);
    }
    to {
        opacity: 1; 
        transform: scale(1) translateY(0);
    }
}

.vertical-tabs-container .content:not(.hidden) {
    animation: tabFadeIn 0.3s cubic-bezier(0.16, 1, 0.3, 1);
}

/* Responsive adjustment for mobile devices (under 768px) */
@media screen and (max-width: 768px) {
    .vertical-tabs-container {
        flex-direction: column;
        min-height: initial;
    }

    .vertical-tabs-container .zavihek {
        width: 100%; 
        border-right: none;
        border-bottom: 1px solid var(--border-color);
        flex-direction: row;
        flex-wrap: wrap;
        padding: 8px;
        gap: 2px;
    }

    .vertical-tabs-container .links {
        flex: 1 1 auto;
        text-align: center;
        padding: 10px 14px;
        font-size: 0.9rem;
    }

    .vertical-tabs-container .content {
        padding: 25px 20px;
    }
}

// 1. CRUCIAL: We bind the openSub function to the global "window" object so the inline HTML onclick can see it!
window.openSub = function(evt, id) {
    const allTabs = document.querySelectorAll('.vertical-tabs-container .links');
    const allContents = document.querySelectorAll('.vertical-tabs-container .content');

    if (allTabs.length === 0 || allContents.length === 0) return;

    allTabs.forEach(tab => tab.classList.remove('active'));
    allContents.forEach(content => content.classList.add('hidden'));

    evt.currentTarget.classList.add('active');
    
    const targetContent = document.getElementById(id);
    if (targetContent) {
        targetContent.classList.remove('hidden');
    }
}

// Safely trigger the first tab after the DOM is fully constructed and loaded
document.addEventListener('DOMContentLoaded', () => {
    const firstTab = document.querySelector('.vertical-tabs-container .links');
    if (firstTab && !firstTab.classList.contains('active')) {
        firstTab.click();
    }
});

Modern Vertical Tabs with Animations