<h3 class="title abs-title">
  <span class="title-main">Main Title Here</span>
  <span class="title-note">This is absolutely positioned.</span>
</h3>

<h3 class="title float-title">
  <span class="title-main">Main Title Here</span>
  <span class="title-note">This subtitle is floated.</span>
</h3>

<h3 class="title table-title">
  <span class="title-main">Main Title Here</span>
  <span class="title-note">I am a table cell.</span>
</h3>

<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;
}

.abs-title {
  position: relative;
  .title-note {
    position: absolute;
    bottom: 2px;
    right: 0;
  }
}

.float-title {
  .title-note {
    float: right;
    position: relative;
    top: 12px;
  }
}

.table-title {
  display: table;
  width: 100%;
  > span {
    display: table-cell;
    white-space: nowrap;
  }
  .title-main {
    width: 99%;
  }
}

.flex-title {
  display: flex;
  align-items: flex-end;
  flex-wrap: wrap;
  > span {
    white-space: nowrap;
  }
  .title-main {
    flex-grow: 1;
  }
}