A simple, plain, responsive, vertical timeline built with CSS and HTML unordered list.

01. How to use it:

<div class="timeline">
  <ul>
    <li>
      <div class="content">
        <h3>Event 1</h3>
        <p>Event 1 Description</p>
      </div>
      <div class="point"></div>
      <div class="date">
        <h4>January 2020</h4>
      </div>
    </li>
    <li>
      <div class="content">
        <h3>Event 2</h3>
        <p>Event 2 Description</p>
      </div>
      <div class="point"></div>
      <div class="date">
        <h4>July 2019</h4>
      </div>
    </li>
    <li>
      <div class="content">
        <h3>Event 3</h3>
        <p>Event 3 Description</p>
      </div>
      <div class="point"></div>
      <div class="date">
        <h4>March 2019</h4>
      </div>
    </li>
    ...
  </ul>
</div>

02. The main CSS styles for the timeline:

.timeline {
    position: relative;
    margin: 0 auto;
    width: 90%;
    
}

.timeline ul li {
    margin-bottom: 50px;
    list-style-type: none;
    display: flex;
    flex-direction: row;
    align-items: center;
}

.point {
    min-width: 20px;
    height: 20px;
    background-color: #be9fe1;
    border-radius: 100%;
    z-index: 2;
    border: 3px #333333 solid;
    position: relative;
    left: 1px;
}

.timeline ul li .content {
    width: 50%;
    padding: 0 20px;
}

.timeline ul li:nth-child(odd) .content {
    padding-left: 0;
}

.timeline ul li:nth-child(odd) .date {
    padding-right: 0;
}

.timeline ul li:nth-child(even) .content {
    padding-right: 0;
}

.timeline ul li:nth-child(even) .date {
    padding-left: 0;
}

.timeline ul li .date {
    width: 50%;
    padding: 0 20px;
    font-weight: normal;
}

.timeline ul li .date h4 {
    background-color: #e1ccec;
    width: 100px;
    text-align: center;
    padding: 5px 10px;
    border-radius: 10px;
}

.timeline ul li .content h3 {
    padding: 10px 20px;
    background-color: #be9fe1;
    margin-bottom: 0;
    text-align: center;
    border-top-left-radius: 10px;
    border-top-right-radius: 10px;
}

.timeline ul li .content p {
    padding: 10px 20px;
    background-color: #e1d9ec;
    margin-top: 0;
    text-align: center;
    border-bottom-left-radius: 10px;
    border-bottom-right-radius: 10px;
}

.timeline ul li:nth-child(even) {
    flex-direction: row-reverse;
}

.timeline ul li:nth-child(even) .date h4 {
    float: right
}

.timeline::before {
    content: "";
    position: absolute;
    height: 100%;
    width: 3px;
    left: 50%;
    background-color: #333333;
}

03. Make the timeline fully responsive:

@media (max-width: 800px) {
    .point {
        min-width: 15px;
        height: 15px;
    }

    html, body {
        font-size: 15px;
    }

}

@media (max-width: 650px) {
    html, body {
        font-size: 14px;
    }
    
    .point {
        min-width: 12px;
        height: 12px;
    }

}

@media (max-width: 450px) {
    html, body {
        font-size: 10px;
    }

    p {
        padding: 10px !important;
    }
}

Demo: Link

Download: Link

Css CSS3 Development HTML & CSS