Modernizr is an open-source JavaScript library designed to facilitate cross-browser compatibility by detecting support for HTML5 and CSS3 features in users' browsers and providing fallbacks for those that do not support these features natively. Launched in 2009 by Faruk Ateş, Modernizr employs feature detection rather than unreliable browser detection methods, allowing developers to tailor user experiences based on the capabilities of the browser rather than the browser type itself. This library is widely used, with over half a million websites in the US alone, and it offers more than 270 tests to ensure modern web technologies can be implemented effectively. Modernizr adds CSS classes to the HTML element based on feature support, allowing developers to apply specific styles or load alternative scripts for unsupported features. While native CSS feature queries are gaining popularity, Modernizr remains a valuable tool for achieving cross-browser compatibility, especially in older browsers that lack support for modern features. Additionally, although Modernizr's load function and yesnope.js have been deprecated, alternative methods like jQuery's getScript() function can be used to load polyfills or scripts conditionally.