No error on missing namespace declaration

Steps to reproduce:
1. Create a new .css file. Using <fx:Style source="xxx.css"/> to import styles to the main application file.
2. The .css file have the following as content:
h|Button {
color:#aaaaaa;
}
3. Save the .css file

Actual Results:
Two warnings:
The CSS type selector 'Button' was not processed, because the type was not used in the application.
Type 'Button' in CSS selector 'Button' must be qualified with a namespace.

Expected Results:
Report error because the namespace prefix, h, is not defined.

Workaround (if any):