I generally do wireframes in photoshop, but you might like to take a look at http://balsamiq.com/ which is a mockup / wireframing tool with a "sketch" style. It's more focussed at UI mockups than specifically CSS I think.
Maybe I read the question wrong, but I thought perhaps it was seeking a tool to output a line-drawing representation of existing CSS and markup. If so, the Fire Fox Web Developer toolbar can help.
Under the Outline option you can have it outline different sections of your page to see clearly how all the elements of your page are nested together. I have found this extremely helpful when trying to troubleshoot a complex layout that someone else has written.