Seems similar to http://webputty.net , though WebPutty has CSS hosting, better integration with existing sites, and compatibility with SCSS.
That aside, what problem do you see this as solving? As in, why would somebody learning CSS by "playing around" (as you say on your About page) want to use LabCSS instead of Firebug or the Chrome debugger?
First of all, webputty will not always be free.
Second thing is, webputty is a website for CSS manipulation, whereas labcss is a snippet website for creating styled HTML elemments. The process of building the website is the significat advantage of this website, and not only the CSS tweaking.
That aside, what problem do you see this as solving? As in, why would somebody learning CSS by "playing around" (as you say on your About page) want to use LabCSS instead of Firebug or the Chrome debugger?