* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
    font-family: 'Arial', sans-serif;
}

body {
    background-color: #f0f4f8;
    display: flex;
    justify-content: center;
    align-items: flex-start;
    padding: 20px;
}

.container {
    display: flex;
    width: 80%;
    max-width: 1200px;
}

.sidebar {
    width: 200px;
    background-color: #0288d1;
    color: white;
    padding: 20px;
    border-radius: 10px;
    margin-right: 20px;
}

.sidebar h2 {
    text-align: center;
    margin-bottom: 20px;
}

.filter-btn {
    display: block;
    width: 100%;
    padding: 10px;
    margin-bottom: 10px;
    background-color: #039be5;
    color: white;
    border: none;
    border-radius: 5px;
    cursor: pointer;
    text-align: center;
    transition: background-color 0.3s;
}

.filter-btn:hover {
    background-color: #0277bd;
}

.filter-btn.active {
    background-color: #ff7043;
}

.timeline {
    flex: 1;
    background-color: #ffffff;
    padding: 20px;
    border-radius: 10px;
    box-shadow: 0 0 15px rgba(0, 0, 0, 0.1);
    position: relative;
}

.timeline h1 {
    text-align: center;
    margin-bottom: 20px;
    color: #1976d2;
    font-size: 1.8em;
    font-weight: bold;
}

#milestone-list {
    list-style: none;
    position: relative;
}

.milestone-item {
    padding: 15px;
    margin-bottom: 15px;
    position: relative;
    display: flex;
    align-items: flex-start;
    transition: transform 0.3s;
}

.milestone-item:hover {
    transform: scale(1.03);
}

.timeline-content {
    display: flex;
    align-items: center;
    width: 100%;
}

.timeline-year {
    background-color: #ff7043;
    color: white;
    padding: 10px 20px;
    border-radius: 50%;
    margin-right: 20px;
    font-weight: bold;
    text-align: center;
    flex-shrink: 0;
}

.timeline-details {
    background-color: #e3f2fd;
    padding: 15px;
    border-left: 5px solid #0288d1;
    border-radius: 5px;
    width: 100%;
}

.timeline-details h3 {
    margin-bottom: 5px;
    color: #1976d2;
}

.timeline-details p {
    color: #333;
}

.paper-link {
    color: #d32f2f;
    text-decoration: none;
    font-weight: bold;
}

.paper-link:hover {
    text-decoration: underline;
}

.timeline:before {
    content: '';
    position: absolute;
    top: 0;
    bottom: 0;
    left: 50px;
    width: 5px;
    background-color: #0288d1;
}