Use W3C documentation for the XHTML1.1 table model. (http://www.w3.org/TR/xhtml-modularization/abstract_modules.html#s_tablemodule)
Defines what elements can occur within the <thead> element in tables.
<!ENTITY % thead.content "( %tr.qname; )+" >
( tr )+
JATS-xhtml-table-1.mod