RecentChanges tables should use the TH scope or headers attributes to identify what the cell is about

The reason is to conform to the guidelines:

• "Use the scope attribute to associate table labels (th cells) with columns or rows."
• "Use the header and id elements to associate table labels (th cells) with individual cells in complex tables."

See also http://www.w3.org/TR/2004/WD-xhtml2-20040722/mod-tables.html#non-visual-rendering

The pb is in RecentChanges.xml, line 573.