モーダルウィンドウ：シンプル｜Modal Window: Simple

<div class="container">
    <button class="modal-button">Simple Modal</button>
    <div class="mask"></div>
    <div class="modal">
        <div class="modal-content">
            <p>【Simple Modal】</p>
            <p>A basic and user-friendly modal window. Simple design that fits well with any website.</p>
            <div class="close-btn-container">
                <button class="close-btn">Close</button>
            </div>
        </div>
    </div>
</div>

body {
    font-family: Arial, sans-serif;
    padding: 20px;
    background: #f5f5f5;
}

.container {
    max-width: 800px;
    margin: 0 auto;
    text-align: center;
}

.modal-button {
    padding: 12px 24px;
    background: #007bff;
    color: #fff;
    border: none;
    border-radius: 6px;
    font-size: 16px;
    cursor: pointer;
    transition: background 0.3s ease;
}

.modal-button:hover {
    background: #0056b3;
}

.mask {
    position: fixed;
    top: 0;
    left: 0;
    width: 100%;
    height: 100%;
    background: rgba(0, 0, 0, 0.5);
    z-index: 1000;
    opacity: 0;
    visibility: hidden;
    transition: all 0.3s ease;
}

.mask.appear {
    opacity: 1;
    visibility: visible;
}

.modal {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%);
    background: #fff;
    padding: 30px;
    border-radius: 8px;
    z-index: 1001;
    opacity: 0;
    visibility: hidden;
    transition: all 0.3s ease;
    max-width: 500px;
    width: 90%;
    transform: translate(-50%, -50%) scale(0.9);
}

.modal.appear {
    opacity: 1;
    visibility: visible;
    transform: translate(-50%, -50%) scale(1);
}

.modal p {
    color: #666;
    line-height: 1.6;
    margin-bottom: 20px;
}

.close-btn {
    padding: 10px 20px;
    background: #6c757d;
    color: #fff;
    border: none;
    border-radius: 4px;
    cursor: pointer;
    transition: background 0.3s ease;
}

.close-btn:hover {
    background: #545b62;
}

/* レスポンシブ対応 */
@media (max-width: 768px) {
    .modal {
        width: 95%;
        padding: 20px;
    }
                
    .modal p {
        font-size: 14px;
    }
    
    .modal-button {
        font-size: 14px;
        padding: 10px 20px;
    }
}

document.addEventListener('DOMContentLoaded', function() {
    const modalBtn = document.querySelector('.modal-button');
    const mask = document.querySelector('.mask');
    const modal = document.querySelector('.modal');
    const closeBtn = document.querySelector('.close-btn');
    
    // モーダルを開く
    modalBtn.addEventListener('click', () => {
        mask.classList.add('appear');
        modal.classList.add('appear');
    });
    
    // マスクをクリックして閉じる
    mask.addEventListener('click', () => {
        mask.classList.remove('appear');
        modal.classList.remove('appear');
    });
    
    // 閉じるボタンをクリックして閉じる
    closeBtn.addEventListener('click', () => {
        mask.classList.remove('appear');
        modal.classList.remove('appear');
    });
});