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[Diving into the code](https://github.com/open-feature/playground/blob/main/packages/ui/src/app/app.tsx#:~:text=evaluateUiFlags), you may notice that an `after` hook has been defined. [Hooks](https://openfeature.dev/docs/reference/concepts/hooks) are a powerful feature that can be used to extend OpenFeature capabilities. In this case, the code is expecting a valid css hex color value. However, _the person configuring the feature flag in a remote feature flag management tool may not be aware of this requirement_. That's where a validation hook could be used to ensure only valid CSS values are returned. In this hook, the evaluated value is tested against a regular expression. If it doesn't match, a warning messaged is logged and the hook throws an error. OpenFeature will catch the error and return the default value. |