I think you can also think of alternative architectures. Sometimes something can be done in another way much more easier. If the producer of HTML file is you, then you can write an HTTP handler to create an Excel document on the server (which is much more easier than in JavaScript) and send a file to the client. If you receive that HTML file from somewhere (like an HTML version of a report), then you still can use a server side language like C# or PHP to create the Excel file still very easily. I mean, you may have other ways too. :)