I have worked with the data shared by you and have rendered a PNG image using below code snippet with Aspose.Html for .NET 18.1, but the image is not being rendered as per your requirements Html_18.1.png. I have logged a request for implementation of this feature. The issue ID HTMLNET-888, has been linked with this thread so that you will receive notification as soon as the issue is resolved.
Aspose.Html.Rendering.Image.ImageRenderingOptions pdf_options = new Aspose.Html.Rendering.Image.ImageRenderingOptions();
// Instantiate PdfDevice object while passing PdfRenderingOptions and resultant file path as arguments
using (Aspose.Html.Rendering.Image.ImageDevice pdf_device = new Aspose.Html.Rendering.Image.ImageDevice(pdf_options, dataDir + "Html_18.1.png"))
// Create HtmlRenderer object
using (Aspose.Html.Rendering.HtmlRenderer renderer = new Aspose.Html.Rendering.HtmlRenderer())
// Create HtmlDocument instance while passing path of already created HTML file
using (Aspose.Html.HTMLDocument html_document = new Aspose.Html.HTMLDocument(dataDir+@"input_test.html"))
// Render the output using HtmlRenderer
We are sorry for the inconvenience.