I went looking for a simple parser example. Sheetjs's "complete" implementation is 18k loc: https://github.com/SheetJS/js-xlsx/blob/master/xlsx.js

But at least the work has been done for me.

A rather more manageable read can be found at https://github.com/woahdae/simple_xlsx_reader/blob/master/li... which seems to illuminate the basics of the xlsx format in exactly the way I was hoping.