Css to the children nodes based on the number of siblings they have

Some one ask me "how can style element based the number of children it has?"

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.

/* 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%;
}