the wro4j library responsibility is to create bundles of css/js resources, it does not generate markup (HTML).
There are two ways of generating resources: build-time (maven or other build tools) or runtime (filter/servlet).
Though it is possible to use it programmatically (you can look at how the maven plugin is implemented - see resources below), it is not that straightforward.
It might be a good feature to evolve the library to become more API friendly and I would like to consider in the future (depending on my spare time though... )
A few useful resources:
Let me know if that helps,