If you're looking to target WPF using C#, there is a tool available that can convert between HTML/CSS and XAML. While it may not cover every possible scenario, it appears to work well for basic HTML/CSS.
Personally, I advise against using this tool for prototyping and instead recommend utilizing WPF or Silverlight along with tools like Expression Blend (or Visual Studio). These tools already have robust prototyping features, making browser-based HTML mockups unnecessary. It's also important to consider whether such a conversion would accurately preserve all desired behavior quirks and nuances.
Prototyping is crucial for establishing assumptions about the UI interactions you plan to support. Using the wrong tool can unnecessarily complicate this process. For quickly presenting a potential UI layout to a client, options like Visio or paper mockups may be more effective as premature interactivity could hinder meaningful discussions about user experience. Avoid getting caught up in details like fonts, colors, and pixels during these discussions.