This concept comes from HTML5 Boilerplate's technique of loading jQuery from a CDN with a local fallback. First, we attempt to load Modernizr from Cloudflare but in the event that requests fails we fallback to a local copy. This is done using the classic try…catch statement. If Modernizr failed to load from Cloudflare our try statement will fail given at that time Modernizr === undefined. Our catch statement would then execute and inject our local copy to the DOM using document.write().