Pseudo-class :only-child

The :only-child CSS pseudo-class represents an
element without any siblings. This is the same as
:first-child:last-child or :nth-child(1):nth-last-child(1),
but with a lower specificity.

li:only-child {
  color: fuchsia;
}

b:only-child {
  text-decoration: underline;
}