EXCEL XML Format is not really standard?

From: http://www.codeproject.com/KB/office/excelxmlreport.aspx

<Cell>
  <Data ss:Type="Number">25</Data>
</Cell>

As far as I'm concerned, that's valid XML. However Excel 2003 complain开发者_开发百科s when it opens up that XML document with a message in a dialog box, "Problems During Load".

Why does Microsoft doesn't really conform to XML ?

Update: I don't understand how http://www.microsoft.com/downloads/en/details.aspx?FamilyId=FE118952-3547-420A-A412-00A2662442D9&displaylang=en can require one single line xml style ?

Excel supports XML-based files, but the file must conform to some rules (excel XML sheet XSD schema).

Look at this example:

Try export a sample excel sheet to an XML file to preview the structure.

Here is Office 2003 XML Reference Schemas Set.