The Running Documents window will appear docked on the right edge of the IDE. The name of the page, "WebForm1.aspx", will appear in the window. Double-click the page and the HTML rendered to the browser when the ASPX page was evaluated by the server will be shown in a code window on the left. The HTML rendered by the ASPX page can be seen by double-clicking the page in the Running Documents window.
The code provided by the Running Documents window is where a developer can set breakpoints and step through the code, just like in server-side code. Stop the project and bring up the form designer.