How to properly clear these floated divs?

Question

I thought I understood the :after method of clearfixing with CSS - but I can't seem to get it to work properly. For some reason the siblingContainer that comes after the buttonContainer containing the floated elements doesn't seem to be clearing?

HTML:

<div class="buttonContainer">
    <div class="action_button">Button 1</div>
    <div class="action_button">Button 2</div>
</div>
<div class="siblingContainer"></div>

CSS:

.action_button {
    font-family: Arial;
    font-size: 9pt;
    font-weight: bold;
    background-color: #f2c059;
    color: white;
    border-radius: 5px;
    display: inline-block;
    padding: 5px;
    cursor: pointer;
}
.buttonContainer {
    margin: 5px;
}
.buttonContainer div {
    margin: 0 5px;
    float: right;
}
.buttonContainer div:last-child:after {
    content: "";
    display: block;
    clear: both;
}
.siblingContainer {
    height: 500px;
    background: gray;
}

fiddle: http://jsfiddle.net/np0c6feh/1/

Answer 1

Can also be done by clearing the parent dic rather than the child.

Instead of

.buttonContainer div:last-child:after {
  content: "";
  display: block;
  clear: both;
}

Use

.buttonContainer:after {
  content: "";
  display: block;
  clear: both;
}

http://jsfiddle.net/AgentStephens/np0c6feh/3/