Now that you've pulled this code out of the old `FontVariationDescription` class, should we still have some unit tests that cover the behavior of it? It seems dangerous to add it here without adding related tests for `FontdeckScript`. But on the other hand, the `FontdeckScript` tests don't really seem like the right place to unit test this type of code in a detailed way. Maybe this should be encapsulated as a class-method on `webfont.Font`?