Conditional CSS: if sibling's child div is present, then - css

Question

In the html fragment below, I want the "main" div to have a background image only if "menu" div is not present in the markup. Is this possible?
<div class="header">
<div class="siteTitle">site title</div>
<div class="tagline">site tagline</div>
<div class='menu'></div>
</div>
<div class="main"></div>

Answer 1

http://www.w3.org/TR/css3-selectors/
E + F Matches any F element immediately preceded by a sibling element E.
E:not(s) an E element that does not match simple selector s
edit :not uses a simple selector, so unfortunately you can't use it to filter by properties of children, only attributes of the element.
A simple selector is either a type selector, universal selector, attribute selector, class selector, ID selector, or pseudo-class.
You could however put a .empty class on the menu and still use it.
.header .menu:not(.empty) + .main {
background:pink;
}
This solution is the best of both worlds, javascript but using css as per normal.
javascript:
if ($('.menu').length == 0){
$('body').addClass('no_menu');
}
css :
body.no_menu .main{
background:pink;
}

Answer 2

The only pure css solution i see is only possible if you rearrange your html like so:
<div class="header">
<div class="siteTitle">site title</div>
<div class="tagline">site tagline</div>
</div>
<div class="menu"></div>
<div class="main"></div>
then you can use this css to only apply a property):
.menu { background: none }
.menu ~ .main{ background: url() } /* or .menu + .main if they are guaranteed to be adjacent to each other on the code */
in this example, you can see it at work: http://jsfiddle.net/tYhxr/
(test it by deleting the menu div and running it again)
check Keyo's asnwer for a link about how selectors work.
If you can't change the html, the javascript is the way to go.
I hope this helps.

Answer 3

You could add a second class to your main <div> that only serves to add the background you want. Then when you create the markup, you just add the second class specifier to the <div> if you need it, or omit it if you don't.
div.main {
//main stuff
}
div.mainbg {
background: *background-specifications*;
}
When your menu div is present, you use this:
<div class="main mainbg">
And when it's missing, you stick with:
<div class="main">