I have the same problem as many here, but i DO HAVE xmlns:android and the xml=... tags. Still I have the same error. I don't wan't just click the ignore instead of warning in the settings so hence the question here posted.
The source is:
<?xml version="1.0" encoding="utf-8"?>
<GridLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
</GridLayout>
The error:
"No grammar constraints (DTD or XML schema) detected for the document. grid_layout_test2.xml /SomeProject/res/layout line 1 XML Problem"
For me it was a Problem with character encoding and unix filemode running eclipse on Windows:
Just marked the complete code, cutted and pasted it back (in short: CtrlA-CtrlX-CtrlV) and everything was fine - no more "No grammar constraints..." warnings