So, my website has a header and a div containing Revolution Slider immediately after it. I'm trying to add a box-shadow below the header - and above the slider. But it doesn't work, unless I also add `margin-bottom` to the header - but that renders the whole exercise moot.

This is the code:

```css
#header {
  display:block;
  min-height: 99px;
  background: #FFFFFF;
  border-top: 3px solid #8dddcd;
  border-bottom: 1px solid #ecf0f1;
  line-height: 99px;
  box-shadow: 0 10px 10px 10px rgba(0,0,0,0.3);
}
#rev {
  position: relative;
}
```

```html
<div id="header"></div>
<div id="rev">the slider</div>
```

Could someone help me figure out what's causing this?