> > I've tried something like this (which XSLT doesn't seem to like):
> 
> How does it express its displeasure?

I've worked it out for myself:
> > 
> >     <xsl:template  match="table">
> >             <xsl:for-each-group  select="tr"
> >                 group-starting-with="td/@colspan=5">

The value of group-starting-with must be a pattern. You probably want
td[@colspan=5]

Next time, do please try to save your readers some time and effort by
quoting the error message. You're not there to set us exam questions.

Michael Kay
http://www.saxonica.com/