The most convenient way to achieve xlsx to html conversion, i.e. export data from Excel spreadsheet (.xls, .xlsx) to HTML format (.htm, .html) would be to use Excel and its File ► Save as.. function. A viable alternative presents itself in some dedicated data exporters or converters, but using a spreadsheet editors seems like the easiest way to do this.
Open XLSX file Open HTML fileA dbf editing and converting tool
One of the many available document converters