Modernizr is a JavaScript library that detects HTML5 and CSS3 features in the user’s browser. Modernizr makes it easy for you to write conditional JavaScript and CSS to handle each situation, whether a browser supports a feature or not. It’s perfect for doing progressive enhancement easily. Modernizr runs quickly on page load to detect features; it then creates a JavaScript object with the results, and adds classes to the html element for you to key your CSS on. Modernizr supports dozens of tests, and optionally includes YepNope.js for conditional loading of external .js and .css resources.