Modernizr is a JavaScript library that detects the availability of various HTML5 and CSS3 features in the user's browser. It does this by running a series of tests on page load and then exposing the results as properties on a global `Modernizr` object, as well as adding classes to the `<html>` element. For example, if the browser supports 'geolocation', it might add a 'geolocation' class, or 'no-geolocation' if not. This allows web developers to write conditional JavaScript and CSS code based on specific feature support, enabling progressive enhancement and graceful degradation. It helps ensure that websites function optimally across different browsers and devices, providing fallbacks or alternative experiences when certain modern features are not available, without making external network requests for its core functionality.