Modernizr
Modernizr is a JavaScript library that detects HTML5 and CSS3 features in a user's browser, allowing developers to write conditional code based on browser support. It runs automatically on page load, adding classes to the HTML element that indicate which features are supported or unsupported. This enables progressive enhancement by letting developers provide fallbacks or alternative experiences for older browsers.
Developers should use Modernizr when building websites that need to support a wide range of browsers, especially when using modern web technologies like CSS Grid, Flexbox, or ES6+ JavaScript features. It's essential for creating cross-browser compatible applications without relying on user-agent sniffing, as it provides a reliable way to detect feature support and implement polyfills or graceful degradation. Use cases include responsive web design, accessibility improvements, and ensuring consistent functionality across different devices and browser versions.