css 子元素按照父元素位置定位
 
```
#parentDiv { position:relative; }
#childDiv { position:absolute; left:50px; top:20px; }
```
css 子元素按照window位置定位( use top:, left:, right:, and bottom: to position as you see fit.)
```
#yourDiv { position:fixed; bottom:40px; right:40px; }
```
