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We can also use Modernizr in our JavaScript to provide some fallback if the visitor’s browser lacks support for any of the HTML5 elements you use.
When Modernizr runs, as well as adding all those classes to your <html> element, it will also create a global JavaScript object that you can use to test for feature support. The object is called, appropriately enough, Modernizr. This object contains a property for every HTML5 feature.