Modernizr

Check which features are present in the browser, and if missing help you load a polyfill.

Does not provide the polyfills.

Checks the features by running them one by one and seeing if the browser gives a sensible response instead of something bad like undefined.

CSS

Special classes are given that only take effect if a feature is missing, e.g. no-svg.

Javascript

Features are detected on a boolean dictionary of features.