Hi dev,
 For Dutch Web Guidelines validity a<th>  have to contains scope
 attribute (scope="col" or scope="row") which means that default wiki
 tables are invalid currently.
 See 
http://www.w3.org/TR/xhtml2/mod-tables.html#non-visual-rendering
 for example of scope attribute use.
 I propose to modify XHTML renderer table header cell to do the following:
 * if a scope is explicitly provided reuse it
 * if there is no scope:
 ** if first line or not first column:<th scope="col">
 ** if not first line and first column:<th scope="row">
 I thing that covers most of the use cases and it will avoid having to
 set explicitly scope attribute in all our standard pages.
 WDYT ?