Continuing from yesterday, the NetBeans features helping you with CSS include some more hidden goodies. Look in the Editor tab, in the Hints section, and you'll see that within your HTML files, helpful suggestions are available for CSS references:
By default, the hints that are shown are not shown as errors. I prefer them to be errors, so configured them as such in the Options window tab shown above, which means that when I reference a class that has not been defined in my CSS files, I get a very clear notification that there's a problem:
When I press Alt-Enter in the above situation (or click the yellow lightbulb in the left sidebar), the available CSS stylesheets are shown and I can choose in which of them a skeleton CSS rule should be generated, which I can then fill out with details.