:nth-of-type()

:nth-of-type(<An+B> | even | odd) {
  /* ... */
}

<div>
  <div>This element isn't counted.</div>
  <p>1st paragraph.</p>
  <p class="fancy">2nd paragraph.</p>
  <div>This element isn't counted.</div>
  <p class="fancy">3rd paragraph.</p>
  <p>4th paragraph.</p>
</div>

/* Odd paragraphs */
p:nth-of-type(2n + 1) {
  color: red;
}

/* Even paragraphs */
p:nth-of-type(2n) {
  color: blue;
}

/* First paragraph */
p:nth-of-type(1) {
  font-weight: bold;
}

/* This will match the 3rd paragraph as it will match elements which are 2n+1 AND have a class of fancy.
The second paragraph has a class of fancy but is not matched as it is not :nth-of-type(2n+1) */
p.fancy:nth-of-type(2n + 1) {
  text-decoration: underline;
}