This type of destinations is not supported and leads to InvalidCastException when importing such elements.

Currently, Telerik Document Processing is available only for .NET Framework and (partially) for Silverlight. Create version targeting .NET Core.

Update: There is a version of Telerik Document Processing compatible with .NET Core 3.0 in the UI for WPF suite. You can use this version in .NET Core projects running on Windows. The assemblies can be downloaded from telerik.com, the Downloads section of UI for WPF -> Betas tab. Please, note that you should have a valid license for UI for WPF in order to be able to see this version. In case you don't, you can use the Trial license by downloading a trial version of the suite. More information about this version of UI for WPF and how it can be used is available in the related blog post.

If HTML document is imported, and it contains image with invalid URL, then the image is imported with this URL in the document model. On subsequent export to Docx, the library tries to download the image data, which throws WebException. Instead, the image should be replaced with generic 'error' image.

Workaround: Manually test the image URL for correctness on HTML import, and replace the data:

        static void Main(string[] args)
        {
            HtmlFormatProvider htmlFormatProvider = new HtmlFormatProvider();
            htmlFormatProvider.ImportSettings.LoadFromUri += (sender, e) =>
            {
                if (!IsValid(e.Uri))
                {
                    e.SetData(File.ReadAllBytes("no-image.png"));
                }
            };
        }

        private static bool IsValid(string uri)
        {
            try
            {
                using (WebClient client = new WebClient())
                {
                    client.DownloadData(uri);
                }
            }
            catch (WebException)
            {
                return false;
            }

            return true;
        }

The same issue appears when exporting to PDF.