<h3 class="title flex-title">
  <span class="title-main">Main Title Here</span>
  <span class="title-note">This is a good look, right here.</span>
</h3>

body {
  padding: 100px;
  font-size: 21px;
}

.title {
  border-bottom: 1px solid #ccc;
  max-width: 500px;
  margin: 40px auto;
}
.title-note {
  font-size: 60%;
  color: #999;
}

.flex-title {
  display: flex;
  align-items: flex-end;
  flex-wrap: wrap;
  .title-note {
    margin-left: auto;
  }
}