[css] CSS: how to add white space before element's content?

None of the following code works :

p:before { content: " "; }
p:before { content: " "; }

How do I add white space before element's content ?

Note: I need to color the border-left and the margin-left for semantic use and use the space as colorless margin. :)

Since you are looking for adding space between elements you may need something as simple as a margin-left or padding-left. Here are examples of both http://jsfiddle.net/BGHqn/3/

This will add 10 pixels to the left of the paragraph element

p {
    margin-left: 10px;
 }

or if you just want some padding within your paragraph element

p {
    padding-left: 10px;
}

/* Most Accurate Setting if you only want
   to do this with CSS Pseudo Element */
p:before { 
   content: "\00a0";
   padding-right: 5px; /* If you need more space b/w contents */
}

Don't fart around with inserting spaces. For one, older versions of IE won't know what you're talking about. Besides that, though, there are cleaner ways in general.

For colorless indents, use the text-indent property.

p { text-indent: 1em; }

JSFiddle demo

Edit:

If you want the space to be colored, you might consider adding a thick left border to the first letter. (I'd almost-but-not-quite say "instead", because the indent can be an issue if you use both. But it feels dirty to me to rely solely on the border to indent.) You can specify how far away, and how wide, the color is using the first letter's left margin/padding/border width.

p:first-letter { border-left: 1em solid red; }

Demo