I've never been working close to UI during my career, neither as Win32 developer nor as Web developer. I was focused on business logic and architecture rather than user interfaces, moreover developing UI was not the best things that I liked to do.
As soon as you good C# developer, understand ASP.net/MVC, RDBMS, enterprise applications design, your values is less, than you lack a HTML/CSS when you do web. I've never treated HTML/CSS as important skill, thinking as I needed I could learn it in 1 day. I was wrong, it not so easy as I thought. Of cause, they are declarative, with no logic, conditions, runtime, but they are languages and requires respect. Sure, it is not so difficult to start with some HTML/CSS than with C#/ASP.net, but as always - experience matters.
I want to improve my knowledge in this area, now I understand that I really need it.