Hello Travis and Zhiyong,
You are right about the ScrollViewer - an element with no explicitly set size would not stretch within a ScrollViewer since it provides infinitely large space to its children. however, if you need the HtmlPlaceholder to be stretched you can set the HorizontalScrollBarVisibility and VerticalScrollBarVisibility properties to False, which will stretch horizontally and vertically accordingly the content of the ScrollViewer.
Also, you can either set explicit size to the HtmlPlaceholder when used in a ScrollViewer, or not place it in a ScrollViewer, since if there is not enough available space for rendering its content, the HtmlPlaceholder will automatically show its built-in ScrollViewer.
All the best,
the Telerik team
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