Use Html5/CSS3 as native mobile/web development. This will allow you to get same result on mobile as well as desktop devices.
IfAs a .NET developer you aremight be familiar with ASP.NET framework development, thus i would advicesuggest to use ASP.NET MVC 4 for cross-platform mobile development by having compatiblewith views/pages in HTML5/CSS3. There are good number of tools and mobile frameworks that are a part of echothis development community, which are using it. Have a
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