I've just checked my site with the HTML validator of W3C: http://validator.w3.org/
Unfortunately there is one small error coming from the contact form plugin, that could be corrected easily:
Error Line 212, Column 30: required attribute "type" not specified
The attribute given above is required for an element that you've used, but you have omitted it. For instance, in most HTML and XHTML document types the "type" attribute is required on the "script" element and the "alt" attribute is required for the "img" element.
I would be very thankful if the author could add this attribute with the next version to make the validation successful.