Hi,
We develop quite a few websites with Drupal and pride ourselves on ensuring that these sites validate on both http://validator.w3.org and http://jigsaw.w3.org/css-validator. We constantly have problems with pages not validating because modules have used the same class ID's or most commonly the same ID for submit buttons.
It would be great if Drupal guidelines could get module creators to run these tools over thier modules first. If modules used their name in the class id's that they use this would solve the problem (some modules do this).
It would be a big boost to drupal if generally sites were validating because modules and drupal were created with just a little regard for this. I don't know how awareness of this can be highlighted to module creators but this post is a first attempt to do so!
Kind regards,
Ryan