I've read many articles to resolve my issue but still not being able to display the modal popup horizontally centered. I have added the following directive but didn't help.
Initially, popup is displayed on the left edge of the page but it centers automatically the moment I expand or shrink the page since I have added RepositionMode="RepositionOnWindowResize" to the extender.
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">