W3C Compliance on Joomla website

I have a Joomla 2.5 website and there are currently 42 errors and 38 warnings, many of which appear to come from plugins. I am looking for someone to address these errors in order that my site becomes 100% W3C compliant, without disabling any plugins or existing functionality. All work would need to be completed on a test site created by the developer and not the live site.

There are also a number of JS errors which may be related to this issue. Views, suggestions please.