In fact, response.renderCSSReference("css/main.css") was the first thing I tried, but it did'nt work.
Now I see that the real condition to reproduce the issue is using a wicket filter url pattern different than /*. I've updated issue summary, description and quickstart to reflect it. In quickstart, I changed wicket filter mapping like this:
Now context path is /*, HomePage is not mounted, so application url is http://localhost:8080/wicket.
Now I use response.renderCSSReference("css/main.css") as should be.
You will see that generated href is css/main.css, wich points to http://localhost:8080/wicket/css/main.css, when it should be ../css/main.css to point to http://localhost:8080/css/main.css, because css path passed to renderCSSReference should be context-relative path, and not wicket-filter-url-pattern relative, isn't it?.