I have created a couple of JavaScript add-ins for Office, but to date, have only done so for Word, so have no experience with using the Excel JavaScript API. I am not certain whether there is any JavaScript API for Excel that will return the markup, including formatting, for the selected range. It appears that you can get, for instance, the fill color, the font color, wrapping, etc. for a range.
http://dev.office.com/docs/add-ins/excel/excel-add-ins-javascript-programming-overview?product=excel
But the getSelectedDataSync method (using coercion type of OOXML) only works for Word.
http://dev.office.com/reference/add-ins/shared/document.getselecteddataasync
I don’t believe at this point, you can get the OOXML markup. You have to get the formatting using the APIs.