Not removing empty tags

Some image replacement techniques use tricks like empty spans
<span></span>.  Tidy is dropping thoes.  I'm failing to find the config
option for this.

I'm really after formatting instead of rewriting any markup.  I trust our
designers to write correct markup but we just want to standardize how the
markup is organized (indented, wrapped, which tags are on a line alone,
etc.).

Thanks,

-- 
Bill Moseley
moseley@hank.org