Building a web page which looks a lot like an Excel sheet is fairly easy if you can specify ahead of time how many columns and rows.
Getting it to calculate cell values based on changes in other cell values takes a bit of work.
Saving results to the user's local disk in anything other than Internet Explorer is nearly impossible. Saving the results to a server requires a program on the server to catch and save it, and eventually send it back for another pass.
What, specifically, would you like?