Learning Web Design as a Form Designer

I am trying to learn ASP.NET (C#) and the biggest struggle I am having is the design aspects of it. I have primarily been a windows form designer. All of the projects I have worked on have used windows forms and it was much more intuitive and cleaner to design in than HTML/CSS. I am trying to learn how to design web pages, but it seems all "design" needs to be done in CSS. How do you design something you don't see and just use pixels and references to other objects? I don't see how you design anything that way.

What is the best way for me to be able to effectively design for websites using CSS and ASP.NET coming from a more visual approach of windows forms? I have read 3 books on ASP.NET and it makes sense but all of the design is done is source views and CSS sheets. Is there another method or approach I should be aware of? I am using VS 2015 BTW as my IDE.

Please and thanks