To get rid of “No grammar constraints (DTD or XML schema) detected for the document.” warning message
Go to the Preferences window, then XML ->
XML Files ->
Then in the Validating files section of the preferences panel on the right,
choose Ignore in the drop down box for the “No grammar specified” preference.
You may need to close the file and then reopen it to make the warning go away.
credits to Marc Bernstein, here is ref link
hope it helps somebody