SpreadsheetML

SpreadsheetML, XML Spreadsheet 2003, or XMLSS, is an XML-based format which was introduced in Microsoft Office 2003 as one of the formats which could be chosen in the "Save As" feature to save spreadsheets, though not the default format (which was XLS, a proprietary binary format). This is a different format from the XLSX format introduced in Office 2007, which consists of a ZIP archive of various files including XML. In contrast, SpreadsheetML is a single XML file, uncompressed, and is unable to store all features which can be present in an Excel spreadsheet.

Opening this file format in OpenOffice apparently requires a Java runtime environment.

Identification
A SpreadsheetML file has the following header:

 <?mso-application progid="Excel.Sheet"?>  

Links

 * Wikipedia article
 * XML Spreadsheet Reference (Microsoft)
 * G-Cloud and open file formats, a cautionary tale