This is something you should specify within the question itself. The tag was tried on Stack Overflow with the best of intentions, but has become little more than a soupy mess today - filtering on that specific tag yields nothing of specific value. Many of those questions have nothing else in common but belonging to that tag.
The same would happen here, filtering on os-agnostic would yield image editors, CD burners, CAD programs and probably anything written in Java - it doesn't have any real value in taxonomy as far as the site goes.
Now, the fact that you need a portable solution should definitely be part of your question, where you indicate just how important that particular constraint actually is. It probably doesn't belong as a tag.