/* styles.css */

body {
  font-family: Arial, sans-serif;
  margin: 0;
  padding: 20px;
}

.chat-container {
  max-width: 600px;
  margin: 0 auto;
  background-color: #fff;
  border-radius: 10px;
  box-shadow: 0 0 10px rgba(0, 0, 0, 0.1);
  padding: 20px;
}

.message {
  position: relative;
  padding: 10px 20px 10px 60px;
  /* Adjust padding to make space for the avatar */
  border-radius: 5px;
  margin-bottom: 10px;
}

.message.reply {}

.avatar {
  position: absolute;
  top: 0;
  left: 10px;
  width: 40px;
  height: 40px;
  border-radius: 50%;
  border: 2px solid #ccc;
}

.message-content {
  position: relative;
  z-index: 2;
}

.replies {
  margin-top: 30px;
  margin-left: 60px;
  /* Align nested replies with avatars */
}

.message::before {
  content: '';
  position: absolute;
  left: 30px;
  /* Align with the center of the avatar */
  top: 0;
  bottom: 0;
  width: 1px;
  background: #ccc;
}

.message.reply::before {
  background: #aaa;
}

.message.reply::after {
  content: '';
  position: absolute;
  left: -90px;
  /* Align with the center of the avatar */
  top: 30px;
  /* Start from the center of the avatar */
  width: 100px;
  /* Length of the horizontal line */
  height: 1px;
  background: #aaa;
}

<link href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap/5.0.2/css/bootstrap.min.css" rel="stylesheet" />
<div class="chat-container">
  <div class="message" id="message-1">
    <img src="https://picsum.photos/40/40" alt="User 1" class="avatar">
    <div class="message-content">Hello, this is the first message.</div>
    <div class="replies">
      <div class="message reply" id="reply-1-1">
        <img src="https://picsum.photos/40/40" alt="User 2" class="avatar">
        <div class="message-content">This is a reply to the first message.</div>
        <div class="replies">
          <div class="message reply" id="reply-1-1-1">
            <img src="https://picsum.photos/40/40" alt="User 3" class="avatar">
            <div class="message-content">This is a nested reply.</div>
          </div>
        </div>
      </div>
      <div class="message reply" id="reply-1-2">
        <img src="https://picsum.photos/40/40" alt="User 4" class="avatar">
        <div class="message-content">Another reply to the first message.</div>
      </div>
    </div>
  </div>
</div>