<p class="paragraph">這是一段文字。</p>

.paragraph {
  font-size: 18px;
  color: #333;
  position: relative;
}

.paragraph::after {
  content: '';
  display: block;
  width: 3rem;
  height: 0.1rem;
  background-color: #007BFF;
  margin-top: 10px;
}