SOURCE

<p class="demo1">
  H <sub>2</sub> O, water water relatively position
</p>

<p class="demo2">
  H <sub>2</sub> O, water water static position
</p>

p{
  border: 1px solid #000;
  margin: 10px auto;
}
sub{
  font-size: 80%;
}
p.demo1 sub{
  position:relative;
  bottom: -.25em;
  vertical-align:baseline;
}
p.demo2 sub{
  position:static;
  vertical-align: sub;
}