Agreed, you would never sell "CSS" to a client...Rather, you might state that you can turn around projects much faster than your competitors because of your vast experience, as well as your skills in employing some advanced technology...blablabla. You could also add that your use of this mysterious advanced technology allows for web pages to be "lighter", hence faster for their customers to view, and accessible to users with disabilities, which also helps with search engines reading/indexing their website, etc. So, yeah, would never sell "CSS" directly. ;-)