Despite the [informal]table 'frame' attribute is set to none (table.borders.with.css = 1), the individual cells still have their borders. I've fixed this in my local copy of the xhtml/table.xsl file (entry|entrytbl template modifying the rowsep/colsepvariables):
<!-- two new conditions -->
I didn't succeed in placing this code directly to my customization layer because of priority issues. I wasn't able to fix it using common scenarios. I'd be grateful for fixing it (or using another approach).
DocBook XSL 1.78.0 / Saxon 6.5.5 / DocBook 4.4