Can CSS detect the number of children an element has?

Question

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) {
/* -or- li:only-child { */
    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%;
}

The trick is to select the first child when it’s also the nth-from-the-last child. This effectively selects based on the number of siblings.

Credit for this technique goes to André Luís (discovered) & Lea Verou (refined).

Don’t you just love CSS3? 😄

CodePen Example:

https://codepen.io/mattlubner-the-decoder/pen/ExaQZQR

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)

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