2.2.2 [CSS-Level2-2009] Section 4.1.5, At-rules
The specification states:
At-rules start with an at-keyword, an '@' character followed immediately by an identifier (for example, '@import', '@page').
Quirks Mode and IE7 Mode (All Versions)
At-rules that start with the at character (@) followed immediately by spaces, tabs, numbers, dashes, or other special characters are ignored and do not allow the actual keyword command to be executed.