Web browsers don't have native support for WordML. In what context are you trying to do this? Converting WordML to HTML would be a fairly complex process because the formatting in a Word document can be scattered across multiple XML files and tracing through multiple levels of style inheretence and correctly interpretting the style is certainly not simple.
If the end user has Word, then attempting to open the document in a web browser will result in an embedded Word window within the browser. Word also natively support outputting its documents in HTML format, so that's another option.
If you really need WordML to be converted to HTML, your best bet is probably licensing a third party solution, though in my experience most of them are far from perfect despite being extremely expensive.