I'm developing printing solution for MS Office 2007. Office automation is not right for me, because it requires Office to be installed. Open XML Document Viewer is solution for converting Word files (.docx) to HTML format by XSLT transform, but it works only for .docx. Can the same technology be used for Excel spreadsheets files?
If you're looking at Excel 97-03 (xls) or Excel 2007 (xlsx) files then I'd recommend FlexCel. I've used it, is very good and honestly quite cheap compared to it's competition.
Note that it doesn't fully support all formatting present in Excel 2007 yet I don't think. But it does have built in functionality to export to HTML.
You could write a SpreadsheetML parser. The schema is available online from Microsoft.
I wrote one a while back that covered data, structure and basic formatting to throw it throw a library and re-save it as an XLS file. Wasn't too difficult.
You could use this article XSL transformation of SpreadsheetML to HTML as a starting point to develop your own transform. You can also look at the open source XSLTs in OpenXML/ODF Translator Add-ins for Office to get some ideas on things you may need to account for in any conversion outside of OOXML. The one thing to keep in mind is that SpreadsheetML is more similiar to PresentationML than it is to WordprocessingML in file structure inside the package (i.e. for every sheet, there is a seperate file).
If your doing this from .NET, I'd do this from LINQ instead of XSLT. I've done transforms from DrawingML into SVG and Linq makes it easy (in terms of similiar functionality to XSLT, staying within .NET, etc.)