afaik respectively understand httrack, you cannot exclude the html files,
because they give httrack the "path" to follow the webpage and locate files.
it' not like on your computer harddisk with a directory listing where you can
search for files you wanna have.
so you'll have to include html and xml and exclude the rest

-* +*.xml +*.html

and then afterwards you can manually delete the html files.

xavier, correct me if i'm wrong