The open-source xls.js library is built for the BIFF-based XLS format, and the XML-based Excel 2003-2004 format. Note that this is not the Open XML formats seen in Excel 2007-2013. So what exactly are these? Does it mean that all/some .xls Excel 2003 files are XML-based? Or does this apply to some other type of Excel document? (templates, macros, scripts)
The wikipedia entry further states that there is loss of fidelity when using such XML formats for spreadsheet storage, which was corrected in the 2007 formats, so is there a prompt asking the user to confirm if he would like to use XML or binary? Or are all documents created in Excel 2003 automatically XML? And for such XML documents is the extension .xml?
Office 2003 was the first version supporting some kind of rudimentary XML format, now superseded with the new Office 2007+ format.
Excel 2003 would open/save spreadsheets in XML format. Those files are still using the XLS extension.
Refer to this blog for some more details
Contrary to OOX where data and metadata are stored in a multipart archive, an Excel workbook file in SpreadsheetML 2003 format consists in a single XML instance, and therefore easily managed ...
The blog post has also these two links to microsoft about SpreadsheetML
To quote from the first link
SpreadsheetML is an XML dialect developed by Microsoft to represent the information in an Excel workbook. SpreadsheetML allows you to save Excel workbooks as XML documents and to open them in Excel. Microsoft created a format that allows you to save, in an XML-based file, almost every Excel customization (including formulas, data, and formatting).