Web Service that returns HTML???

Firstly I'm new to Web Services and ASP.NET etc, I am however an experienced C# and C++ developer. I therefore apologise in advance if this is a stupid question... :) I want to build a Server type application that does some processing and returns a HTML document. This processing is probably along the lines of get some data, apply XSLT = HTML. I want customers to be able to build their own websites around this server. A WebService seems like the perfect thing here because ultimately I want both Think Client (Browser), and Thick Client (C# Form) to be able to access the server. My question is; if I write a web service that returns HTML, how does the consuming client then just display this. It seems to me that I want something that is between a webservice and a web server??? I realise that this is a pretty basic question but I've waded through a number of technical articles but these always seem to imply some knowledge at the client side about how to display the information... Any help is welcome, especially any suggestions of good articles or books.