I set the background color of the outer div to blue but it is not displayed. Why?

A demo of it:

```css
.question-template {
    width: auto;
    height: auto;
    background-color: blue;
}
.question {
    float: left;
    margin: 10px;
}
.participants {
    background-color: green;
    float: left;
    margin: 10px;
}
```

```html
<body>
<div class="question-template">
    <div class="participants">234</div>
    <div class="question">Some lorem ipsum text</div>
</div>
</body>
```

[jsFiddle](http://jsfiddle.net/4zjtp/)