The Modernizr GitHub Library is being used on 25,000+ websites.

Modernizr is a service that compiles JavaScript files, which can then be used to run tests on browsers on what features they have available.

It's useful for detecting new browser features, older browsers that may not be compatible with your app, or to display custom notifications or alerts to users who may be running outdated browsers or clients.

With Modernizr, you can compile your own custom set of "tests", based on what features your website or web app require to function properly.