HTML to Excel conversion is very complex thing. Excel to HTML is relatively much easier. Not every type of HTML can be converted to Excel.
HTML is a markup language and rendered by Web Browsers. Just check the size of Google Chrome, FireFox, Internet Explorer etc. They are almost more than 60 MB and they are full of composite DLLs etc. On the other hand, Aspose.Cells is a single DLL and its size is around 10 MB.
In short, free form HTML (i.e. HTML not generated by Microsoft Excel) is very hard to convert to Excel file.
Besides, it is out of Aspose.Cells scope to convert HTML to Excel. Because it deals with Excel files not HTML files. These are just additional features.
However, we have already logged your issue (CELLSJAVA-42332) in our database for product team consideration and investigation. Once, it will evaluate your issue, we will be in better position to help you.
I will suggest you, first create a simplest Excel file, then save it to HTML using Microsoft Excel menu commands and then observe what tags or markups like (h1, h2, div etc.) are being generated by Microsoft Excel, then use those tags to generate your HTML. In this way, you will be able to generate the HTML dynamically which is easily understandable by Aspose.Cells and you will also not have to wait for this issue to be fixed. Thanks for your understanding and have a good day.