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 <th> element in tables.
<!ENTITY % th.content "( #PCDATA | %Flow.mix; )*" >
( #PCDATA | email | ext-link | uri | hr | inline-supplementary-material | related-article | related-object | disp-formula | disp-formula-group | break | element-citation | mixed-citation | nlm-citation | bold | italic | monospace | overline | roman | sans-serif | sc | strike | underline | chem-struct | inline-formula | def-list | list | mml:math | abbrev | named-content | styled-content | graphic | media | preformat | inline-graphic | private-char | fn | target | xref | sub | sup )*
JATS-xhtml-table-1.mod