Hi EVOgroup,
You’ll notice that others have reported this as an issue in these forums. You’ll also find that if you search Google for this W3C validator error you’ll find many experienced web developers discussing this across the Google support forums, W3.org forums and HTML forums. The general consensus appears to be that the validator is wrong or out of date. The advent of Schema.org markup means that there is new markup that is not viewed by the validator as correct but everyone generally accepts that it is correct and that you can use these attributes. Search engines such as Google want this markup in this format and because we are in the business of developing an SEO plugin if Google likes to see the itemprop description added to the meta description then we’re going to add it. It should be noted that we added this because many of our users asked for it, we get requests all the time to add more support for Schema.org because that’s what Google and other search engines want.
However, because of the feedback from users who want to keep their HTML compliant with the strict HTML standards we will be adding a feature in the next release where you can deactivate the Schema.org markup that All in One SEO Pack adds.
