adjacent hyphens within comment 

Cause:

This is due to too many hyphens "-" in the declaration of a comment. A comment is defined like this <!-- comment -->

Solution:

Remove the additional hyphens

BAD   <!--- comment --->
GOOD  <!-- comment -->

References:

W3C spec : http://www.w3.org/TR/html401/intro/sgmltut.html#h-3.2.4