Q:

nth-child skipping over specific class less

I need to select the nth child of a class not counting elements of a specific class in less. As an example, given:
li
li class="skip_this"
li
li
li
I want to make the nth-child skip the skip_this class when counting, meaning that if I wanted the 3rd li that is not skip_this, it would actually select the 4th li, because it wouldn't count the one with skip_this.

A:

Use :not() like
li:nth-child(2n):not(.skip_class){

}

li:nth-child(2n):not(.skip_class){
    background:green;
    }
<ul>
 
   <li>test</li>
   <li class="skip_class">test</li>
   <li>test</li>
   <li>test</li>
   <li>test</li>
   <li>test</li>
  </ul>

Update

My intention is that if I wanted to select the 2nd child that is not 'skip_class', it would select the third child, since the 2nd child is a 'skip_class'

Use the immediate sibling selector + like
li:nth-child(2n):not(.skip_class), .skip_class + :not(.skip_class)

li:nth-child(2n):not(.skip_class),
.skip_class +:not(.skip_class) {
  background: green;
}
<ul>

  <li>test</li>
  <li class="skip_class">test</li>
  <li>test</li>
  <li>test</li>
  <li>test</li>
  <li>test</li>
</ul>

Udpate
To make it clear, trying to target a set of elements with a condition(class) irrespective of some siblings does not work because nth-child and nth-of-type will target all siblings or siblings of the same type.
When you add a condition(:not(.skip_class)), it works independently of the the other selector(nth-child)
See Can I combine :nth-child() or :nth-of-type() with an arbitrary selector?

