Use (ex) length for typography.

When typographical elements have any box model declarations on either top or bottom use ex unit instead of em.

The ex takes into account the fonts x-height, therefore for is better for using on top and bottom box model declaration.

Also I made a little [thing](http://jsbin.com/AFeVowAD/1). to demonstrate the use of ex units. 

As a follow on I would suggest:

```
	dl,
	dir,
	menu,
	ol,
	pre,
	plaintext,
	ul,
	xmp,
	p {
		margin: 2ex 0
	}
```

###Browser Support
IE7+
Chrome 11+
Firefox 1+
Opera 8+
Safari 5.1+