at opensourcecms.com textpattern and cms made simple are the ones that can produce valid xhtml, provided tha your template is.
You can use the search function at CMS Matrix and tick the checkbox for XHTML compliant. There will be too many results with only that criteria so narrow the results down by the CMS features you are looking for in addition to XHTML compliance.
WordPress. Drupal (Pretty sure that outputs XHTML)... Either way, just get a decent template developed for whatever system you plan to use and make sure it's XHTML valid.