I discovered that the $macros.ContactForm() produces a contact from that is not xhtml 1.0 strict.

To fix this I have edited the _utility > forms > contact.view file as follows

Changed:

<div class="form_field"><textarea name="message" id="message" rows="7" tabindex="4"></textarea></div>

To:

<div class="form_field"><textarea name="message" id="message" rows="7" cols="" tabindex="4"></textarea></div>

I also believe the following line should not exist as the form element is missing. Therefore I removed it without side effects.

<div class="form_field_name"><label for="url">Name</label></div>