CSS :before and :first-child combined

Try

#navigation_center li:first-child:before {
    content: '';
}

Edit: I wanted to expand on this answer with comments made by FelipeAls. The original question used :first which is not a valid CSS selector. Instead, use :first-child. Also the order of the pseudo-selectors is important. The first child selector must come first.

I tend to think of :before as a kind of modifier to a selector. It does not actually select an element only the space just before the selected element.

Although hradac's answer should do the trick i thought it would be best to run through some possible permutations to help newcommers.

.works:first-child:before
{
	color: green;
	content: 'working ';
}
.works:not(:first-child):after
{
	color: green;
	content: ' is working';
}


.broken:before:first-child
{
	color: red;
	content: 'this will never show up';
}
.broken:after:not(:first-child)
{
	color: red;
	content: 'and this will not show up either';
}

works:
<div>
	<div class='works'>
	 something
	</div>
	<div class='works'>
	 something
	</div>
	<div class='works'>
	 something
	</div>
</div>
<hr/>
broken:
<div>
	<div class='broken'>
	 something
	</div>
	<div class='broken'>
	 something
	</div>
	<div class='broken'>
	 something
	</div>
</div>

Let's take this apart:

• Three div.works are inside a div
• Three div.broken are also inside a div
• The first rule of CSS adds a green text "working " before. It does so by selecting the first-child and then selecting the empty space right before it.
• The second rule adds " is working" after each block that comes after first, by analogy it first selects each block that doesn't fall under the first-child definition, and then selects the empty space before them.
• The following two rules, will not find a block to attack themselves to. The :before:first-child attempts to select an empty space, but then tests if it is a first-child and it is not (since technically it's not yet in the DOM tree), the similar problem is with :not(:first-child).