It looks like by AddOutputFilterByType directive somehow overrides the
defaults, and also there are a few more types that could usefully be
compressd. So now it lists all the compressible file types in the
directory.
DirectoryIndex index.xhtml
AddType application/xhtml+xml;charset=UTF-8 .xhtml
AddType font/otf;outlines=CFF .otf
-AddOutputFilterByType DEFLATE application/xhtml+xml font/otf
+AddOutputFilterByType DEFLATE application/xhtml+xml text/css font/otf \
+ text/plain text/x-csrc application/postscript