Use Html5/CSS3 as native mobile/web development. This will allow you to get same result on mobile as well as desktop devices.
As a .NET developer you might be familiar with ASP.NET framework development, thus i would suggest to use ASP.NET MVC 4 for cross-platform mobile development with views/pages in HTML5/CSS3. There are good number of tools and mobile frameworks that are a part this development community.
You may look for more details @ What’s New for Mobile Development in ASP.NET MVC 4.
There is also possibility to use Silverlight, but i did not explore it yet. Here is a refernce to look - Silverlight for mobile phones