StackOverflow 18626938
HTML
<div>
<div class="e">e</div>
<div class="f">f</div>
<div class="f">g</div>
</div>
<div>
<div class="e">e</div>
</div>
<div>
<div class="f">f</div>
</div>
CSS
div > div {
background-color: cyan;
}
div div:only-child {
background-color: transparent;
}