jsFiddle is the perfect solution; it provides a custom environment (based on user selections) to test (or fiddle with) your JavaScript, HTML, and CSS ... code in the appropriate areas of the page.

But CSS Exfil is not an infallible method. It can only steal HTML attributes found on the page at load time, and not from dynamically injected code after the initial page load. Gualtieri argues ...

The latest example is a web skimmer that uses CSS code to blend within the pages of a compromised store and to steal customers' personal and payment information. By hiding their payment info ...