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HTML 

              
                <!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Nested JS Tabs</title>
    <!-- Basic Styles for the Tabs -->
    <style>
        body {
            font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
            padding: 2rem;
            background-color: #f4f7f6;
        }

        .tabs-container {
            max-width: 600px;
            margin: 0 auto 2rem auto; /* Add margin-bottom to separate tab groups */
            background: #fff;
            border-radius: 8px;
            box-shadow: 0 4px 12px rgba(0, 0, 0, 0.1);
            overflow: hidden;
        }
        
        /* Style for nested tabs */
        .tab-content .tabs-container {
            max-width: 100%;
            margin-top: 1rem;
            box-shadow: inset 0 2px 8px rgba(0,0,0,0.06);
            border: 1px solid #e9ecef;
        }

        .tab-titles {
            display: flex;
            border-bottom: 1px solid #dee2e6;
            background-color: #f8f9fa;
        }

        .tab-title {
            padding: 1rem 1.5rem;
            cursor: pointer;
            border-bottom: 3px solid transparent;
            margin-bottom: -1px; /* Aligns with the container's border */
            transition: all 0.2s ease-in-out;
            font-weight: 500;
            color: #495057;
        }

        .tab-title.active {
            color: #007bff;
            border-bottom-color: #007bff;
            background-color: #fff;
        }

        .tab-contents {
            padding: 1.5rem;
        }

        .tab-content {
            line-height: 1.6;
        }

        /* Utility class to hide elements */
        .hidden {
            display: none;
        }
    </style>
</head>
<body>

    <!-- PARENT TAB GROUP -->
    <!-- Note: The 'active' class is no longer needed in the HTML -->
    <div class="tabs-container">
        <!-- Tab Titles -->
        <div class="tab-titles">
            <div class="tab-title">Outer Tab 1</div>
            <div class="tab-title">Outer Tab 2 (With Nested Tabs)</div>
        </div>
        <!-- Tab Contents -->
        <div class="tab-contents">
            <div class="tab-content">
                <p><strong>Content for Outer Tab 1.</strong> This is the first level of tabs.</p>
            </div>
            <div class="tab-content hidden">
                <p>This is the content for Outer Tab 2.</p>
                
                <!-- NESTED TAB GROUP -->
                <div class="tabs-container">
                    <div class="tab-titles">
                        <div class="tab-title">Inner Tab A</div>
                        <div class="tab-title">Inner Tab B</div>
                    </div>
                    <div class="tab-contents">
                        <div class="tab-content">
                            <p><strong>Content for Inner Tab A.</strong> This tab group is completely independent and nested.</p>
                        </div>
                        <div class="tab-content hidden">
                            <p><strong>Content for Inner Tab B.</strong> Clicking these tabs does not affect the outer tabs.</p>
                        </div>
                    </div>
                </div>

            </div>
        </div>
    </div>


    <script>
        document.addEventListener('DOMContentLoaded', function() {

            /**
             * Initializes a single tab group based on its container.
             * It finds all titles and contents that are DIRECT children and wires them up.
             * @param {HTMLElement} tabContainer - The .tabs-container element.
             */
            function initializeTabs(tabContainer) {
                // Find title and content elements using more specific selectors.
                // ':scope' refers to the 'tabContainer' element itself.
                // This ensures we only get the direct children of the container's
                // '.tab-titles' and '.tab-contents' wrappers, ignoring any nested tabs.
                const titles = tabContainer.querySelectorAll(':scope > .tab-titles > .tab-title');
                const contents = tabContainer.querySelectorAll(':scope > .tab-contents > .tab-content');
                
                if (titles.length !== contents.length) {
                    console.error("Mismatch between tab titles and content panels.", tabContainer);
                    return;
                }

                /**
                 * Switches the visible tab to the one at the specified index.
                 * @param {number} activeIndex - The index of the tab to show.
                 */
                function switchTab(activeIndex) {
                    titles.forEach((title, index) => {
                        title.classList.toggle('active', index === activeIndex);
                    });
                    contents.forEach((content, index) => {
                        content.classList.toggle('hidden', index !== activeIndex);
                    });
                }

                // Add a click event listener to each tab title
                titles.forEach((title, index) => {
                    title.addEventListener('click', () => {
                        switchTab(index);
                    });
                });
                
                // --- INITIALIZE ---
                // Programmatically set the first tab (index 0) as active on load.
                switchTab(0);
            }

            // --- Main Execution ---
            // Find all tab containers on the page and initialize each one independently.
            // The script will run on the outer container first, then on the inner one.
            const allTabContainers = document.querySelectorAll('.tabs-container');
            allTabContainers.forEach(initializeTabs);

        });
    </script>

</body>
</html>
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