You can drop XHTML modules by setting their .module entities to ignore, for instance, the following drops the more advanced features for tables:
<!ENTITY % XHTML1-table.module" "IGNORE">
<!ENTITY % XHTML11-strict
PUBLIC "-//W3C/XHTML 1.1 Strict"//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11-s.dtd">
%XHTML11-strict;