The use of a query string like that hurts/eliminates much of beneficial caching (see e.g.
this article
). For a single small loader file (such as the widgetset loader bootstrap JavaScript) it can be ok, but it might be a bad idea to use it for a large number of larger files.
Unfortunately, truly forcing CSS refresh is hard, in part also because e.g. Chrome sometimes seems to ignore the information the server has given it about what caching it is allowed to do (at least for stylesheets) and caches CSS more aggressively.