PlaceHolder in comment Form
Reading through Placeholder module I come to know this 


  Once FAPI uses HTML5 to render elements, this module can be replaced
  by the "placeholder" attribute available as part of the HTML5 spec.
  You can update the FAPI to use the "placeholder" attribute right now
  by using elements module. This means that this module is only useful
  if you are concerned about older browsers that do not support
  "placeholder".


try using elements module so the support for HTML5 attribute can become available in your installation.

Here is the way to add default placeholder attribute.