On most projects I've been one, designers has produced HTML code, then developers turned it into ASP.NET, including master-pages etc that should really be a part of design.
After it has become ASP.NET, designers could not work on the code with their tools.
I know that a lot of the design of ASP.NET is made with the purpose of separating code and design, and in principle designers should be able to work on design aspects with the Visual Web Developer, but I've never seen a designer using VWD.
How is cooperation done in practice, and what is about the best one can expect from a designer?
I've found that using ASP.NET MVC will make the designer's job much easier. Especially if you stay away from using things like HTML.RenderImage, and instead place an IMG tag on the page with
<%= ViewData["MyImage"] %>as the src. This will allow the designer to see the html they like and understand, while giving you the flexibility to set the source(which is all the developer should be doing). The goal being to stay away from ASP.NET controls, which would confuse a designer, while still keeping their flexibility to develop quickly.