The screenshot tells it all:http://htl10.users.sourceforge.net/FontVal-Linux-xml-tiled.png
For existing Linux users (probably very few besides me...), just grab "mono-webbrowser.dll" from https://github.com/HinTak/Font-Validator/tree/master/bin
and replace the one you had. Provided you have webkit and webkit-sharp installed, it should just work.
There is no code change in FontVal itself - it is all in the mono webbrowser bridge, which is an external project. Windows users don't need to worry about things changing (to the worse?) for them - there isn't any.
While fixing problems with mono's webkit bridge, I learned a great deal about webkit. One thing I learned is that Webkit on Linux is very different from webkit on Mac os X, so the same solution does not work on Mac os X. OTOH, some of that webkit knowledge probably will translate. Let's see.