タブメニュー：アンダーライン｜Tab Menu: Underline

<div class="tab-container">
    <ul>
        <li class="selected" data-id="tab-1">Tab 1</li>
        <li data-id="tab-2">Tab 2</li>
        <li data-id="tab-3">Tab 3</li>
    </ul>
    <div class="tab-content selected" id="tab-1">Good morning. This is the content of Tab 1.</div>
    <div class="tab-content" id="tab-2">Hello. This is the content of Tab 2.</div>
    <div class="tab-content" id="tab-3">Good evening. This is the content of Tab 3.</div>
</div>

.tab-container ul {
    margin: 0;
    padding: 0;
    list-style: none;
    display: flex;
    border-bottom: none;
}

.tab-container ul li {
    flex: 1;
    padding: 16px 24px;
    text-align: center;
    cursor: pointer;
    transition: all 0.3s ease;
    position: relative;
    color: #999;
    font-weight: 300;
    letter-spacing: 0.5px;
}

.tab-container ul li.selected {
    color: #333;
    font-weight: 500;
}

.tab-container ul li.selected::after {
    content: '';
    position: absolute;
    bottom: -1px;
    left: 0;
    right: 0;
    height: 2px;
    background: #333;
    animation: slideIn 0.3s ease;
}

@keyframes slideIn {
    from {
        transform: scaleX(0);
    }
    
    to {
        transform: scaleX(1);
    }
}

.tab-container ul li:not(.selected):hover {
    color: #333;
}

.tab-content {
    display: none;
    padding: 32px 24px;
    min-height: 150px;
    color: #333;
    line-height: 1.6;
}

.tab-content.selected {
    display: block;
    animation: fadeIn 0.3s ease;
}

@keyframes fadeIn {
    from {
        opacity: 0;
        transform: translateY(10px);
    }
    
    to {
        opacity: 1;
        transform: translateY(0);
    }
}

const tabContainers = document.querySelectorAll('.tab-container');
tabContainers.forEach(container => {
    const tabMenuItems = container.querySelectorAll('ul li');
    const tabContents = container.querySelectorAll('.tab-content');
    tabMenuItems.forEach(tabMenuItem => {
        tabMenuItem.addEventListener('click', () => {
            tabMenuItems.forEach(item => item.classList.remove('selected'));
            tabMenuItem.classList.add('selected');
            tabContents.forEach(tabContent => tabContent.classList.remove('selected'));
            document.getElementById(tabMenuItem.dataset.id).classList.add('selected');
        });
    });
});