first-element

by shriek

HTML

<ul>
  <li data-year="2020"> <span>Year 2020 List 1<span> </li>
  <li data-year="2020"> <span>Year 2020 List 2</span> </li>
  <li data-year="2021"> <span>Year 2021 List 1 </span> </li>
  <li data-year="2021"> <span>Year 2021 List 2</span> </li>
  <li data-year="2021"> <span>Year 2021 List 3</span> </li>
</ul>

CSS

[data-year="2020"]:first-of-type {
  color: red;
}

[data-year="2021"]:first-of-type {
  color: green;
}