WebView.CaptureBitmap as per the explanation should capture the full HTML view as bitmap but it is capturing only the view which is visible on the screen. It is not capturing the part which is not visible.
Sub EnableSlowWebViewDocumentDraw
Dim p As Phone
If p.SdkVersion >= 21 Then
Dim jo As JavaObject
jo.InitializeStatic("android.webkit.WebView").RunMethod("enableSlowWholeDocumentDraw", Null)
End If
End Sub