This is due to having an element with a 'mkto...' class nested inside another element with a 'mkto...' class. Below is an example:
<div class="mktEditable">
<div class="mktEditable"></div>
</div>
You will need to remove the 'mkto...' class reference from one of the elements to be able to pass validation and approve the draft as in the corrected example below:
<div class="mktEditable">
<div class="newClassName"></div>
</div>