For example, this allows the selectors to address every other row in a table, and could be used, for example, to alternate the colour of paragraph text in a cycle of four. If n is negative, then start counting from the end of the element. In other words, this matches the nth child of that type after all the children of that type have been split into groups of m elements each. If n is negative, then start counting from the end of the element. Negative numbers mean count from the end of the element.
Negative numbers mean count from the end of the element.
If 'n' is negative, then perform the above calculations but starting from the bottom of the element. If either number is negative, then starts counting from the last line of the block. The range may go either forward or backwards. See the CSS3 UI draft. Something needs to be decided about how pseudo-elements inherit from their surroundings, how their contents inherit from them, and also which properties apply to each.