Time to use Firebug
Mark I am trying desperately to get my head around how to use the information from W3C and Firebug - I have multiple errors! Site now up for 2 years only to realize that you need to run a W3C verification.
On W3C I did a on-site url check, which runs of the whole site and can see what is required, it is how to go about doing the cleaning. Can one edit in Firebug and save the file and upload to WP3 - would you have to rename a file etc.
All the You Tube information available goes through the W3C side, but not on how to treat the file.
Is it safer to download and test each .php .html etc one by one, clean them in something like NoteTab and do the same for CSS?