Conditional CSS doesn't work on Tomcat

Hey all maybe you can help me :) 

I want to use some Conditional CSS in my webapp. 

For test I build in some simple user.agent detection like it ist describe here.

Now my main.css looks like this: 

@if user.agent safari gecko1_8 {
.headerLine {
color: white;
margin: 0px;
/* padding: 5px; */
background-repeat: repeat-x;
background-image: url(../img/head.png);
position: fixed;
z-index: 2;
height: 60px;
}
}

the CSS file is loaded in my webapp.gwt.xml.

The CSS code above works fine aslong I'm working in development mode. But when I compile my project and deploy it to a tomcat this code doesn't work any more :(

Maybe someone can help me with this problem. 

Thanks in Advance 

Dominic 

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