Modernizr tells you what HTML, CSS and JavaScript features the user’s browser has to offer.
What is Modernizr?
It’s a collection of superfast tests – or “detects” as we like to call them – which run as your web page loads, then you can use the results to tailor the experience to the user.
Why do I need it?
All web developers come up against differences between browsers and devices. That’s largely due to different feature sets: the latest versions of the popular browsers can do some awesome things which older browsers can’t – but we still have to support the older ones.
Modernizr makes it easy to deliver tiered experiences: make use of the latest and greatest features in browsers which support them, without leaving less fortunate users high and dry.
Source: https://modernizr.com/