:only-child
jQuery :only-child filter test
by Yohan Yang
HTML
<p>
<span>P > SPAN</span>
<button>P > SPAN + BUTTON</button>
</p>
<p>
<button>P > BUTTON</button>
</p>
CSS
p {border-right: 1px solid red; padding: 10px;}
.on {background-color: yellow; }
JavaScript
$('button:only-child').addClass('on');