There are a number of Excel XML 2003 errors in both reading and writing.
This format is very useful to have functioning as it is extremely easy for a script to generate, takes up little space, and is openable by both Excel and LibreOffice. In referring here to XML files, XML 2003 is meant.

The code below illustrates a number of problems:-
* COLORS - Basic named colours are not recognised (here 'red') but hex (here #00ff00) is. Excel recognises them both.
* DATES - 1999-04-01 is not recognised as a date, but 1999-04-02T00:00:00.000 is. Excel recognises them both. This shorter form of date is very helpful in terms of file size.
* COLUMN ASSIGNMENT - If you remove ss:ExpandedColumnCount="3" and also <Column ss:StyleID="s2"/>  then LibreOffice will not correctly apply the third column even though it has index="3". Excel does so correctly.
* AUTOFIT - The reader is supposed to autofit the AutoFitWidth columns (numbers and dates) but LibreOffice fails to do so. Excel does.
* DATE FORMATS - With <NumberFormat ss:Format="yyyy/mm\-dd;@"/> LibreOffice loads the two separator symbols as / and - respectively which I think is technically sensible, however my Excel loads them differently (the / comes out as -) and this may need to be borne in mind or noted for compatibility reasons.
* LOCKS - Open an XML file in LibreOffice and then close the file (but not LibreOffice) - LibreOffice retains a lock on it and you have to close the entire application to close the lock. It ought to release the lock on closure, or perhaps earlier on finishing reading the file. If the file is read-only, I would suggest releasing the lock when the reading is complete. This should be very easy to fix and would be useful as testing XML output involves closing and opening numerous times and closing the application each time is a pain!
Some further issues -
* CELL BLOAT - Create a one-cell spreadsheet in LibreOffice, save it and read it back. Reading in takes a long time because the export has written a vast number of empty cells/rows, creating a large file. Removing those empty cells, the result is small and LibreOffice reads it in swiftly. Excel does not produce these empty cells when the same task is carried out.
* SAVING - Create a new Calc spreadsheet, put a date in cell A1, format it YYYY-MM-DD from the date format list, then save as M.S. Excel XML 2003, close and reopen that file. The resulting file reads in the date, but not the format.
Cheers,
David

SAMPLE FILE (utf8 file no utf8 BOF marker) -

<?xml version="1.0"?>
<Workbook xmlns="urn:schemas-microsoft-com:office:spreadsheet" xmlns:o = "urn:schemas-microsoft-com:office:office" xmlns:x = "urn:schemas-microsoft-com:office:excel" xmlns:ss = "urn:schemas-microsoft-com:office:spreadsheet" xmlns:html="http://www.w3.org/TR/REC-html40">
<Styles>
<Style ss:ID="s1"><Font ss:Bold="1" ss:Color="#008000"/><NumberFormat ss:Format="0.00_ ;[Red]\-0.00\ "/></Style>
<Style ss:ID="s2"><Font ss:Color="red"/></Style>
<Style ss:ID="s3"><Font ss:Color="blue"/><NumberFormat ss:Format="yyyy/mm\-dd;@"/></Style>
</Styles>
<Worksheet ss:Name="Sheet1">
<Table ss:TopCell="2" ss:LeftCell="2" ss:DefaultColumnWidth="20" ss:ExpandedColumnCount="3">
<Column ss:StyleID="s1" ss:AutoFitWidth="1"/>
<Column ss:StyleID="s2"/>
<Column ss:StyleID="s3" ss:Index="3" ss:AutoFitWidth="1"/>
<Row ss:StyleID="s1">
<Cell ss:Index="2"><Data ss:Type="String">Title</Data></Cell>
</Row>
<Row>
<Cell><Data ss:Type="Number">111</Data></Cell>
<Cell><Data ss:Type="String">aaaa</Data></Cell>
<Cell><Data ss:Type="DateTime">1999-04-01</Data></Cell>
</Row>
<Row>
<Cell><Data ss:Type="Number">2222</Data></Cell>
<Cell><Data ss:Type="String">bbbb</Data></Cell>
<Cell><Data ss:Type="DateTime">1999-04-02T00:00:00.000</Data></Cell>
</Row>
</Table>
</Worksheet>
</Workbook>