First of all, let’s get the essential question answered: what is Modernizr?
Based on the official site, Modernizr is “A JavaScript library that detects HTML5 and CSS3 features in the user’s browser.”
Although HTML5 and CSS3 are great but many of the new features they have, as we already know by now, are not much applicable in old browsers. Modernizr, helps to address this problem by testing the user’s browser on whether it supports a particular feature. If the feature is “unsupported”, then we can deliver an appropriate script or function to imitate the feature it lacks.