Taming Advanced CSS Selectors - CSS Essentials - page 244

In the previous case, the first yellow li element will be the fourth. If you
just want to start counting from the first li element, you can use a simpler
expression:
ul li:nth-child(3n) {
color: yellow;
}
In this case, the first yellow li element will be the third, and every other
third after it. Now imagine you want to target only the first four li elements
within the list:
ul li:nth-child(-n+4) {
color: green;
}
The value of :nth-child can also be defined as “even” or “odd”, which
are the same as using “2n” (every second child) or “2n+1” (every second
child starting from the first), respectively.
:NTH-LAST-CHILD
The :nth-last-child pseudo-class works basically as the :nth-child
pseudo-class, but it starts counting the elements from the last one.
Using one of the examples above:
ul li:nth-child(-n+4) {
color: green;
}
Instead of matching the first four li elements in the list, this selector will
match the last four elements.