body {  
    font-family: 'Arial', sans-serif;  
    margin: 0;  
    padding: 0;  
    background-color: #f4f4f4;  
}  
  .h3{
            font-size:2em;
        }
.timeline-container {  
    width: 100%;  
    max-width: 800px; /* 可选：限制最大宽度以适应PC屏幕 */  
    margin: 0 auto;  
    padding: 20px;  
}  
  
.timeline {  
    position: relative;  
    padding: 20px 0;  
    list-style-type: none;  
}  
  
.timeline:before {  
    content: '';  
    position: absolute;  
    top: 0;  
    bottom: 0;  
    width: 2px;  
    background-color: #000;  
    left: 50%;  
    margin-left: -1px;  
}  
  
.timeline-item {  
    position: relative;  
    width: 50%;  
    padding: 10px 40px;  
    box-sizing: border-box;  
}  
  
.timeline-item:before, .timeline-item:after {  
    content: '';  
    position: absolute;  
    top: 15px;  
    width: 20px;  
    height: 20px;  
    border-radius: 50%;  
    background-color: #fff;  
    left: 2%;  
    margin-left: 1px;  
}  
  
.timeline-item:after {  
    background-color: #fff;  
    border: 2px solid #000;  
    z-index: 1;  
}  
  
.timeline-item:nth-child(even) {  
    float: right;  
    text-align: left;  
}