Ugh. In that case, they're missing the point of Modernizr. If there's no polyfill or fallback available, the solution isn't to say "Don't use Safari". At the very least, they could display as useful message that says something like "Your browser doesn't appear to support feature X, try browser Y instead."

When I see "Don't use Safari", I interpret that as "I'm too lazy to make this work in Safari", rather than "this doesn't work in Safari".