Please read about the updated syntax of CSS variables in the first update of this post.

Since a little bit more than a month (as of the time of writing) there is a Editor’s Draft for a CSS Variables Module by Google (Tab Atkins and Luke Macpherson) and Daniel Glazman. Just a few days ago the Working Draft was updated.
The first draft was written in 2008 by Daniel Glazman but was not added to the official specification. The new WD extends this proposal by Glazman.

Disclaimer: This article is part of a small series about the latest CSS working drafts. The first article in this series was about the CSS Hierarchies Module.

CSS Variables

The CSS Variables Module Level 1 suggests to use variables for data that is reused in stylesheets. These key-value storages provide the possibility to store “meaningless” data as you can do in HTML: Variable keys consist of a data--prefix and the variable name.

Any property name starting with the prefix “data-” is a data property. Data properties are defined to be valid but meaningless as they are meant solely for allowing authors to pass custom data around their page, similar to the custom data attributes in HTML.
~ Defining Variables With Data Properties – CSS Variables Editor’s Draft

Using data- as a prefix for variables is still in discussion and may likely be changed to something more “connected” to variables ($ or &).

Edit: The mailing-list www-style is discussing the use of data and Ian Muir wrote an article about the problem.

Syntax

Variables are defined by mapping them to the :root pseudo-class. The WD does not really explain why the variables must be declared on the root-element. Maybe it’s because the data needs to be bound to an element to not “fly around” in the global scope (which does not really exist in CSS).
Defined variables can also be set on every other element and overwrite the existing value of a variable.
So this is how you set variables.

To get a specific variable, you do not declare the variable-name but you call data as a function on a certain property with the variables name (without data--prefix) as an argument.

An Example

So here is an example:

/* Set data */
:root {
  data-main-color: #bada55;
}
 
/* Get data */
.foobar {
  color: data(main-color);
}

As said before, variables can be overwritten for specific elements. So we can add the declaration of the variable to .foobar:

.foobar {
  data-main-color: #fe57a1;
  color: data(main-color);
}

The color of .foobar is now #fes7a1 and not #bada55.

Real World Examples

Well, as this is an Editor’s Draft and pretty new it’s not being implemented in any browser.

I’ve created a test case on dabblet with vendor prefixes which tests the two examples above. I’ve run this test on the major browser alphas and nightlies.

Even though variables are not natively supported by CSS in any browser yet, we can use a very similar approach: All CSS preprocessors support variables. Here is an example with SASS:

/* Setting a variable */
$main-color: #bada55;
 
/* Get the variable's value */
.foobar {
  color: $main-color;
}

Conclusion

We can use variables with preprocessors today which is pretty rad. As with the CSS Hierarchies Module CSS in the future may support ideas that come from implementations in preprocessors somehow.
I think most developers are happy about these adoptions and will likely use it. But as it will take some time for all vendors to implement this feature – and even more time until browsers that don’t support such new features vanish – we will have to use preprocessors to get variables in our CSS working.

Update 16.04.2012

Since last week the CSS Variables Module Level 1 is an actual Working Draft of the W3C.

It’s now possible to define variables with the var-prefix instead of data which is still pretty complicated. I wonder why they not adept an existing concept as in LESS or SASS.

Thus the data()-function changed accordingly to var().

Furthermore it’s possible to set variables on a certain element but use them in a more complex definition. An example might be the usage as follows (while elements with class important and primary are divs):

.important { var-color: red; }
.primary { var-color: blue; }
div { color: var(color); }

Also nestings for variables are possible:

:root {
  var-color: #ddd;
  var-border: 1px solid var(color);
}

The bright and shiny future

As I think there still are some issues with this Working Draft I like the fast step forward in making this a WD in only a view weeks.
So sooner or later variables will be part of the CSS specification and will be implemented in browsers. I hope this will be very soon.