How to fake non-support of new web features for testing in modern browsers

We are told to use progressive enhancement when building websites that will use modern features if available but still work if not.

However, I find it hard to test the fallback cases. Of course there should be some tests with some older browsers, but it is hard to target non-support of specific features. It is also nicer to use recent developer tools that are not available in legacy browsers.

Example: I use a flexbox layout with a float-based fallback. I would like to debug the fallback in chrome, but flexbox is already available there.

Is there something like "reverse polyfills" that remove support instead of adding it? What other solutions are available?