Actually, that's not true. The [CSS specification](http://www.w3.org/TR/CSS21/syndata.html#rule-sets) states:

> The selector (see also the section on selectors) consists of everything up to (but not including) the first left curly brace ({). A selector always goes together with a declaration block. When a user agent cannot parse the selector (i.e., it is not valid CSS 2.1), it must ignore the selector and the following declaration block (if any) as well.

If the style blocks were combined, then, for example, Chrome would view the `:-moz-full-screen` pseudo-class as invalid and ignore the entire declaration block.