Website Designers specially novice Designers make lot of mistakes and converting such photoshop layout to html/css is very difficult.
This article has lot of useful info about it.
As a designer sending files in for PSD to HTML conversion, you have a big impact on whether your designs will be converted quickly and accurately or late and with errors. During many years in the PSD to HTML industry, we have seen thousands of Photoshop designs ranging from those very easy to work with to the ones which nearly doubled standard PSD to HTML production time! The following is our collection of 10 Photoshop tips for time and cost-efficient PSD to HTML conversion.
You can read complete article here : 10 Photoshop Tips for easy PSD to HTML conversion