That's broken now in Office 2013 and Office 2016. Several of my users -at the same time- found that they could not open "excel" documents provided by several different vendors, all of which were actually HTML tables in files named ".xls". My guess is that Microsoft broke this with a software update, but I've yet to research the problem.
One of our history systems did that, but it doesn't work with the Office 2012.
There is an XML format that worked with the pre-OOXML versions of Office and should continue to work with moderns versions as well. Plus it's a drastically simpler file format than the complex zipped XML-cluster of files that describe XLSX documents.
This has the minor niggle that it will throw a warning to the user ("The file format and extension of *.xls don't match...")
But it has the "advantage" over the csv approach that you can include formatting via hmtl/css styles.