CSS Feature Toggles, developed by Keith Clark, is a free Chrome extension that provides a convenient way for developers to toggle support for selected CSS features in order to test progressive enhancement fallbacks.

With this extension, developers can easily disable the support of modern CSS features directly from the Chrome DevTools. By disabling features like flexbox, grid, or blend-modes, developers can see how their web pages or applications will render in browsers that do not support these features. This is particularly useful for ensuring that the design and layout of a website or app are gracefully degraded for users on older or less capable browsers.

CSS Feature Toggles is a valuable tool for developers who want to ensure their websites and applications provide a consistent and accessible experience across different browsers. It allows them to easily test and debug their code, making it easier to identify and address any compatibility issues that may arise.