Take the following code for instance:
- Hello World
- Hello World
- Hello World
&
The only way you'd be able to get anywhere near to that in pure CSS is to do a selector on either nth-child(odd) or nth-child(even). If you want exactly the last half (and not either odd or even), then you'd have to use JavaScript/jQuery.
Using jQuery, you could get them using:
var yourList = $("ul li");
yourList = yourList.slice(0, Math.floor(yourList.length/2));