We write unit-tests tests for server-side source code and for JavaScript. Even when putting aside benefits of TDD approach it gives us a priceless thing – early problem reporting during refactoring. So you make change, you run tests and you will know at once if anything broke. What about CSS? You encapsulate a declaration set into a separate rule, look through the site pages where dependent classes used and satisfied proceed with refactoring.
What we are going to do is a modal window that opens in the center of the screen by pressing a link while everything that underlies the window becomes blurred. It can be done by [Dialog element](http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#the-dialog-element “Dialog` element”), but these day we don’t need any JavaScript realy to solve this task. Well, there are two main techniques to handle onClick events in CSS – checkbox-hack and selection by :target. The second way binds our modal window to a URL hash.