HtmlHost for Silverlight requires that the Silverlight plug-in's Windowless parameter be set to True. In windowless mode, the Silverlight plug-in does not have its own rendering window. Instead, the plug-in content is displayed directly by the browser window. This enables Silverlight content to visually overlap and blend with HTML content if the plug-in and its content both specify background transparency.

To set the Windowless parameter, you would complete the following steps:

1. Create a Silverlight application that includes a C1HtmlHost control.
2. Navigate to the Solution Explorer, and expand the YourProject.Web node (where YourProject is the name of the project).
3. In the Solution Explorer, double-click the page in which the Silverlight plugin is declared. For example, double-click YourProjectTestPage.aspx or YourProjectTestPage.html file (where YourProject is the name of the project) to open the page.
4. In the page scroll down to the  <div id="silverlightControlHost"> tag and add the following parameter in the list of parameters between the <object></object> tags:

<param name="windowless" value="true" />

5. Save your change and return to the MainPage.xaml page.

When the Windowless parameter is not set to True, you may receive a warning when running the application.