CSS selector to create a CSS style if a parent has one child, and a different one for two children - selector

Question

Is it possible to use pure CSS selectors to give a nodes children (in my case ULs) different properties, in particular height, based on the amount of siblings the child have?
For example, if a node has 1 child, the height of the UL is automatic, however if the node has 2 children, then those children's height is say '200px'?

Answer 1

Are you looking for:
If the list item is the only child, its height will be auto. If there are multiple list items, the list items will all have a height of 200px.
ul li {
height: 200px;
}
ul li:only-child {
height: auto;
}
Just to note, only-child is supported on all browsers except for IE8 and older.
http://reference.sitepoint.com/css/pseudoclass-onlychild

Answer 2

I'm not fully sure about browser compability, but
ul li:first-child:last-child {
height: auto;
}
Works aswell and in the end, makes a whole lot of sence :)

Answer 3

Assuming that the reason of the question is that your UL's are generated dynamically you can use the same code to define an unique ID's to those UL's so the CSS can be applied properly

Answer 4

CSS is not a logical language. You can't use "if this do that" logic. So you should use jQuery for this practice.
EDIT
Here is your required code
var liCount = $("ul li").size(); // How many list items UL' have
var constant = 10; // Constant px Value for calculate new pixel.
$("ul li").css("height",constant*liCount); //Css manipulation
$(".result").html(constant*liCount+"px is the li's height"); // what is the result
<ul>
<li>First List Item</li>
<li>Second List Item</li>
<li>Third List Item</li>
<li>Fourth List Item</li>
</ul>
You can test this
in jsfiddle (i defined height in jquery for seeing changes)
http://jsfiddle.net/guJBt/1/