Now I don't remember why, but I've not added alignment to the TH tag in HTML.

$ ./dist/compare-targets.py html xhtml xhtmls html5 | grep CellOpen 
tableCellOpen       '<TD~A~~S~>'  '<td~a~~s~>'  '<td~a~~s~>'  '<td~a~~s~>'
tableTitleCellOpen  '<TH~S~>'     '<th~s~>'     '<th~s~>'     '<th~s~>'

Note the '~A~' and '~a~' marks. They represent the alignment attribute. They're 
only present in <TD>, not <TH>.

At this point some users may rely in the "table heading cells are not alignable 
in HTML" behavior, but I think we should align it as well.

Comments?