Could you give us a link/example of the pages and errors in question?

With these, they would be checking your main index file.  Of course with Joomla, that might mean you need to edit your templates depending on the theme you are using.  You would then go down the list of errors and see if you can figure out how to fix each one.

Also, just so you don't tear all your hair out - you may not need to validate 100%.  I would just be sure that all browsers on all OS's can see your content and it all looks the same across the board.