I've got from web designers HTML prototype, that I have to implement in ASP.NET web site. Just including the whole HTML with CSS and JS files didn't work well, because the CSS files had global settings, that corrupt existing layout.
So I had to comment and modify CSS selectors to adapt th the site. Not an easy task, because I am not expert in CSS and it took a time to understand quite large set of CSS rules, that the designers used.
Excellent presentation from Damian Edwards at RDN about CSS based layout with Visual Studio 2008 came just in time.
The useful lesson: in a class attrubute we can specify more than one space-separated class names
(e.g. cssClass="navigation active" ) and apply separate rules to each of class names.
A few links that helped me:
CSS2 Specification Selectors
W3C CSS Validator: http://jigsaw.w3.org/css-validator/validator
VS 2008 has a few new Designer windows, that help to work with CSS and JS