In this PnP Webcast, we covered techniques on how to validate SharePoint Framework client-side web part property pane values when user is configuring web part. This is relatively common scenario for client-side web part developers where you want to ensure that the provided input value in the web part property pane is as expected. There's two different ways to achieve this, which are inline validation and remote API based validation. You'd use inline validation for simplistic scenarios like validating string input value based one expected structure. Remote API based validation means a scenario where you call for example oob SharePoint API to validate user input.