Netbeans recognizes CSS classes, when you mention the CSS file in a <link> tag.
Ex. index.jsp
<link href="whatever.css" ...>
<div class="classInWhatever">...</div><!-- classInWhatever is found, no error -->

Problem:
But it doesn't recognize CSS classes, when the CSS file is in a <jsp:include> or a <%@ file include> tag.
<jsp:include page="whatever.css"/>
<div class="classInWhatever>...</div> <!-- Editor shows 'Class classInWhatever is not found' -->

Solution:
The CSS validator should recognize all classes included with <jsp:include> or <%@ file include>.