HTMLTidy clean converts <colgroup <col attributesl to the form class="cx".
Unfortunately only IE and Avant support class attributes on <col tags. All the other browsers ignore them. I pestered Opera for instance and they respondonded that the W3 spec requires them to ignore the class attribute on <col tags. This is idiotic. There SHOULD be a way to apply a class to a column, and properly a different style to table headers and bodies, but the spec is the spec. It wants you to apply classes cell by cell.
I consider this an error in the spec, but the fact remains the code tidy generats will not work generally even if it is conformat HTML. What HTML tidy effectively does is strip out all the <colgroup formatting, rather vandalous activity, that might pass unnoticed long enough that it is extremely difficult to fix.