I want to develop an application for WP7 using HTML,CSS and Javascript something Similar to Nokia WRT(Web Widget) Will this Support in WP7? .Will the app require signing ? Can I use Metro style for WP7?
Windows Phone 7 (and 7.5) applications are written either using the XNA framework (for gaming style applications) or Silverlight (for "apps"). Windows Mobile 6.5 (and prior) have applications either using the compact edition of WinForms, or using embedded C++.
The current design guide for the Silverlight applications for Windows Mobile 7.x is virtually identical to that for Metro on Windows 8 (there are even a bunch of controls to help you out).
As for using a HTML/CSS/JS application; it may work in browser, but I'm not aware of any ways to (nor plans to allow) distribute apps of this type to Windows Mobile 7.x devices.