Well, as stated, the message “No Grammar Constraints (DTD Or XML schema)” is just a warning. But even so, some of us may find it annoying to see that warning icon even though it really does not halt or disrupt the compilation process.
To disable this message in Eclipse, do the following steps:
Go to Windows > Preferences
Go to XML > XML Files > Validation
Select “ignore” in the drop down selection of “No grammar specified”
You may have to clean your project or restart Eclipse if ever the warning icons still persist.