modalpopupextender only centers on resize

CancelControlID - ID of an element that cancels the modal popup.BackgroundCssClass - A CSS class to apply to the background when the modal popup is displayed.

You can take advantage of the rich client framework to easily build client-centric web applications that integrate with any backend data provider and run on most modern browsers. ASP.NET AJAX makes it possible to easily take advantage of AJAX techniques on the web and enables you to create ASP.NET pages with a rich, responsive UI and server communication. And because ASP.NET AJAX is an extension of ASP.NET, it is fully integrated with server-based services. In addition, ASP.NET AJAX offers you the same type of development platform for client-based web pages that ASP.NET offers for server-based pages. This new web development technology from Microsoft integrates cross-browser client script libraries with the ASP.NET 2.0 server-based development framework.

ASP.NET AJAX is a free framework for building a new generation of richer, more interactive, highly personalized cross-browser web applications.

Modalpopupextender only centers on resize