It's the idea of OOCSS.
In fake code, you could do that:
```html
<div id="human"></div>
<div id="monkey"></div>
```
or that
```html
<div class="has-mouth has-fingers has-opposable-thumbs">
<div class="has-mouth has-fingers has-facial-hair">
```
And that way, you can reuse a lot of the CSS.