CSS3: apply styles to child nodes based on the number of siblings they have

Quoting this answer by Lübnah to the question can CSS detect the number of children an element has :

Clarification:
Because of a previous phrasing in the original question, a few SO citizens have raised concerns that this answer could be misleading. Note that, in CSS3, styles cannot be applied to a parent node based on the number of children it has. However, styles can be applied to the children nodes based on the number of siblings they have.

---

Original answer:
Incredibly, this is now possible purely in CSS3.

/* one item */
li:first-child:nth-last-child(1) {
    width: 100%;
}

/* two items */
li:first-child:nth-last-child(2),
li:first-child:nth-last-child(2) ~ li {
    width: 50%;
}

/* three items */
li:first-child:nth-last-child(3),
li:first-child:nth-last-child(3) ~ li {
    width: 33.3333%;
}

/* four items */
li:first-child:nth-last-child(4),
li:first-child:nth-last-child(4) ~ li {
    width: 25%;
}

Credit for this technique goes to André Luís (discovered) & Lea Verou (refined).
Don't you just love CSS3? :)
Sources:

• http://andr3.net/blog/post/142 (André Luís)
• http://lea.verou.me/2011/01/styling-children-based-on-their-number-with-css3/ (Lea Verou)