Modernizr

Modernizr is a JavaScript library that detects support for HTML5 and CSS3 features in a user’s browser. Rather than relying on browser detection, Modernizr runs a series of feature tests and exposes the results as boolean properties on a global Modernizr object and as CSS classes on the document’s root element. For example, if the browser supports flexbox, Modernizr.flexbox will be true and the html element will receive a "flexbox" class; if not, a "no-flexbox" class is added. This allows developers to apply progressive enhancement through CSS selectors like .flexbox and .no-flexbox or to conditionally execute JavaScript and load polyfills when a feature is missing.

Modernizr is typically loaded at the start of a page. A developer can either include the full library or generate a custom build containing only the tests they need via Modernizr’s download builder. It does not polyfill features itself, but it enables the use of polyfills or alternative code paths when a given feature is unavailable.

The project was created by Faruk Ateş and released in the early 2010s as an open-source tool under a permissive license. Over time, Modernizr has evolved with new tests for CSS and HTML features and provides an extensible API for adding custom tests.

Criticism and considerations: feature detection can be imperfect for some APIs, and including a large default build can impact page load time. For many modern development workflows, a minimal, custom Modernizr build or alternative feature-detection strategies may be preferred.