css selectors - Is it possible to select every other group of three in CSS?

Question

Is it possible to select every other group of three in CSS? And if it is; how?

So in the sample below apply style to the 4-6 and 10-12 lis.

<ul>
    <li>1</li>
    <li>2</li>
    <li>3</li>
     <li>4</li>
     <li>5</li>
     <li>6</li>
    <li>7</li>
    <li>8</li>
    <li>9</li>
     <li>10</li>
     <li>11</li>
     <li>12</li>
</ul>

I know [pure] JavaScript and jQuery can do this but I am looking for a pure CSS solution if it exists.

Answer 1

You're looking for nth-child:

ul li:nth-child(6n+4),ul li:nth-child(6n+5),ul li:nth-child(6n+6) {
    background:red;
}

http://jsfiddle.net/bhlaird/utEP4/1/