The web didn't move past it for all the years it required flash to be installed to run games.

Now we are talking about installing a different plugin to run games (but only if you use a certain browser). So this is an improvement on the status quo regarding web games.

I figured someone would complain. The reason is simple -- our users are normal people and there is no reason to suggest that they first compare Opera, Safari, Firefox and Chrome and then select the one that best fit their needs.

I don't think "Best viewed in X" is a problem when you're trying to force people to use standards-compliant, modern browsers.

What made it a problem in the past is that people would rely on weird, kludgy, non-standard behavior of Browser X and slap on a "Best viewed in X".

But it sounds like this is exactly like those kludges.

Instead of detecting which browser the user has and saying "best viewed in X" where X is some other browser, how about detecting which features you need, and then giving a clear error on which were missing? This is better in many ways, for example if the next update to a browser adds the missing features to it, then it will just work on your site.

That would confuse our users when there is no reason to.

What would confuse them? You can customize the warning message to be as clear as you want. The benefit your users get is your page will actually work the second their browser can support it, not when you officially define browser X to be "acceptable".