Writing a cross platform and device independant app by using browser Instances method

We know that when we developing an app we have to choose platform to build. like .NET, Java, PHP etc... So biggest problem we face is we can not make an universal app to suit all these users.

For eg: When we build a Standalone app by using .NET if we want to available it to the android users we have to recode the whole app to that platform. And if we targeting Ipad, Smart phones, Tv screens things are againg different.

But when I do research I found that people are building app mainly targeting to Web. If a user wants the standalone version of it they behind the scene load a browser instance. To avoid user feels that they work on a browser what they are doing is give native features using CSS and other stuff. So user feels that he is using an app specifically build for that platform. But developers knows that behind the scenes they using a browser instance.

So I didn't have experience in this magic. :) I am developing websites by using ASP.NET.

So If I want to do above thing Do I need to code in a specific way? Is this a good and best way?

I think this is a good approach.