Well, I'm not a front-end dev myself, so maybe I overestimate the importance of this service, but here goes:
It can be useful for people who work/experiment with CSS a lot. Normally they would have to change the CSS file, save it, (possibly) upload/move it to a server, reload the page and see the results.
With WebPutty they can change something in CSS and immediately see the result in the preview window. When they're satisfied with the results, they can save the final version (which would be hosted on WebPutty's servers), thus eg making the result ready to use on a production server.
So, maybe it's nothing groundbreaking, just removes some annoying steps that a developer has to do when working with CSS.
Knowing FogCreek and having used their products in the past, I'm pretty sure that the quality will be good and they'll deliver what they promise.
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