I think the Tapestry components should generate valid XHTML 1.0 Strict. Components that generate XHTML can still be used to build HTML 4.01 pages, so XHTML 1.0 Strict seems to be the best choice.
Currently there are the following XHTML issues with Tapestry:
*) some tag attributes are not quoted properly. For example <td
colspan=2> instead of <td colspan="2">.
*) some attributes are no longer supported in XHTML 1.0 Strict. For
example the img tag no longer has a border attribute. This
affects the border parameter of the Image component as well. People who
create HTML 4.01 pages can easily remove the border using Cascading Style Sheets.
*) some tag attributes have changed. For example <script language=...> has
been replaced by <script type=...>