Thanks for your inquiry. Please note that Aspose.Words mimics the same behavior as MS Word does. If you convert your input html to Docx using MS Word, you will get the same output.
Could you please share some detail about ‘z-index property’ which is not reflected in output document? Please manually create your expected Word document using Microsoft Word and attach it here for our reference. We will investigate how you want your final Word output be generated like. We will then provide you more information on this along with code.