How I can get URL of embedded resource(.html) in my app which I can use like a value of WebControl.Source? Or what better way to open embedded resources?
Embedded Resources do not have an URL instead the can be referenced with their namespace. Read about how to use embedded Resources:
http://www.codeproject.com/Articles/3089/Understanding-Embedded-Resources-in-Visual-Studio
There are also multiple questions and answers for this on stackoverflow: