The first-child pseudo-class selector doesn't seem to be having any effect. Here's the CSS, followed by HTML:
.social-block a:first-child {
margin-bottom: 20px;
}
<div class="social-block">
<a href="#" target="_blank"><img src="stylesheets/img/socialblock-facebook.png" alt="socialblock-facebook" width="300" height="125"></a>
<a href="#" target="_blank"><img src="stylesheets/img/socialblock-twitter.png" alt="socialblock-twitter" width="300" height="125"></a>
</div>
Can't tell where I'm going wrong!