Adding CSS styles for the table layout via the TableCssLayoutTemplates adds the style element inside the body, which is non W3C compliant content.
The table layouts are implemented via CSS, so RadEditor adds a <style> tag in the content after the table. The current implementation uses CSS classes and not inline styles, so a <style> element is required for the table appearance. Since a <style> tag must be present, the content cannot be XHTML compliant, because all the HTML RadEditor generates must be self-contained (i.e., RadEditor cannot put styles in the <head> of the page where the HTML will be used, nor generate a stylesheet file).