cdn and fallback

https://www.davepaquette.com/archive/2015/05/06/link-and-script-tag-helpers-in-mvc6.aspx

It is a common approach to reference popular frameworks from hosted CDN in order to reduce network load on your server and potentially improve performance for the end user. For popular frameworks like jQuery and Bootstrap, there is a good chance that the browser will already have a cached version of these files.

Referencing files from a CDN can be a bit of a pain because you also need to provide a fallback to a version of the file hosted on your servers. A fallback is necessary because you do not want your application to go down just because the CDN is not currently reachable.

The script and link tag helpers make it easier to specify a fallback test and file location. Here is a basic example for the popular Bootstrap framework:

<link rel="stylesheet" href="//ajax.aspnetcdn.com/ajax/bootstrap/3.0.0/css/bootstrap.min.css"
      asp-fallback-href="~/lib/bootstrap/css/bootstrap.min.css"
      asp-fallback-test-class="hidden"
      asp-fallback-test-property="visibility"
      asp-fallback-test-value="hidden" />

<script src="//ajax.aspnetcdn.com/ajax/bootstrap/3.0.0/bootstrap.min.js"
        asp-fallback-src="~/lib/bootstrap/js/bootstrap.min.js"
        asp-fallback-test="window.jQuery">
</script>