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Authoring CSS is a complex, time consuming task requiring not only skilled human graphic designers but also skilled human coders. Practice shows that today human authored code is still superior to machine generated CSS, but the code characteristics which make the difference have not been researched or even quantified yet. In this paper we introduce the abstractness factor, a quality metric which reveals the advantages of human authored code and can serve as an optimization criterion and benchmark for automated CSS coding. We argue that a high abstractness factor represents a high maintainability and reusability of the presentation document as well as the content document. By an evaluation of 100,000 HTML pages randomly gathered from the Web we show that today's typical style sheet document has a significantly higher abstractness factor compared to code fully machine generated by state-of-the-art applications.