Identifies a "head" entry in an HTML table.
Processed like an HTML th.
These elements contain th: tr.
The following elements occur in th: abbrev, acronym, action, anchor, application, author, authorinitials, beginpage, calloutlist, caution, citation, citerefentry, citetitle, classname, classsynopsis, cmdsynopsis, code, command, computeroutput, constant, constructorsynopsis, corpauthor, corpcredit, database, destructorsynopsis, email, emphasis, envar, errorcode, errorname, errortext, errortype, exceptionname, fieldsynopsis, filename, firstterm, footnote, footnoteref, foreignphrase, formalpara, funcsynopsis, function, glosslist, glossterm, graphic, guibutton, guiicon, guilabel, guimenu, guimenuitem, guisubmenu, hardware, important, indexterm, inlineequation, inlinegraphic, inlinemediaobject, interface, interfacename, itemizedlist, keycap, keycode, keycombo, keysym, link, literal, literallayout, markup, medialabel, mediaobject, menuchoice, methodname, methodsynopsis, modespec, mousebutton, nonterminal, note, olink, ooclass, ooexception, oointerface, option, optional, orderedlist, orgname, othercredit, para, parameter, personname, phrase, productname, productnumber, programlisting, programlistingco, prompt, property, quote, remark, replaceable, returnvalue, revhistory, screen, screenco, screenshot, segmentedlist, sgmltag, simpara, simplelist, structfield, structname, subscript, superscript, symbol, synopsis, systemitem, tip, token, trademark, type, ulink, uri, userinput, variablelist, varname, warning, wordasword, xref.