Features

Allows HTML5/CSS3 Feature detection.
Custom build for the library available.
Yepnope.js allows fallback to polyfills.
Retrieve prefixed/non-prefixed CSS properties

<html class="no-js>

Gets Replaced with below at runtime:
						
<html class="js flexboxlegacy canvas canvastext webgl no-touch geolocation rgba backgroundsize borderimage borderradius boxshadow textshadow cssanimations csstransforms csstransitions fontface audio svg inlinesvg smil svgclippaths" lang="en">

Usage in JS:
<script>
	var animationsSupported = Modernizr.cssanimations;
</script>

When to use Modernizr?

Almost everytime you want to detect features.
For graceful degradation.
Limiting behavior across browsers.