The WebForms viewer works like the HTML5 Viewer in a client-server pair with the Reporting REST Service. Even if the Reporting REST Service is added in the same WebForms project, the service does not operate in the context of the client project. If you need to use data modified in the WebForms project, it has to be saved in the data source, from where the Reporting REST service can access it.
The HTML5 ASP.NET Web Forms Report Viewer is built on and requires ASP.NET 4 Web Forms or newer framework.
A reference to Telerik.ReportViewer.Html5.WebForms.dll
The WebForms wrapper is designed to request all Kendo UI CSS and JS files, including the viewer's HTML, CSS and JS templates, from the resources of the Reporting REST Service. If you will not add changes in the default styles of the viewer, you need only a link to jQuery of version greater than 1.9.1.