Generate absolutely minimal HTML with the right structure and no
superfluous boilerplate on it. Cut and paste into Excel.
It's easy (really easy). Just don't trap yourself into thinking it must
be hard.

Let Excel do the work for you by using its good abilities to read HTML.
Trying to make a web protocol fit around Excel would be much harder,
especially as Excel doesn't have any well-defined standards for its
formats.

Here are a couple of options if you want to auto-generate an Excel file
from your Java code.  One would be to create a simple CSV download and
then open with Excel.  It would be a basic layout without colors or
formatting.  Another option would be to use the Jakarta POI API.  You
can read and create Excel files using this API.  I've never used it,
but it looks pretty simple:  http://jakarta.apache.org/poi/

-Mike