You can use the fallback mechanism for CDN URLs
In your BundleConfig.cs
For css files
bundles.Add(new StyleBundle("~/Content/css", "http://CDN_URL.vo.msecnd.net/Content/css")
// If it falls, then your website content will be loaded
.IncludeFallback("~/Content/css", "sr-only", "width", "1px")
.Include(
"~/Content/bootstrap.css",
"~/Content/site.css"));
Almost the same for js files
bundles.Add(new ScriptBundle("~/bundles/jquery",
"http://CDN_URL.vo.msecnd.net/bundles/jquery")
// If it falls, then your website content will be loaded
{ CdnFallbackExpression = "window.jquery" }
.Include("~/Scripts/jquery-{version}.js"));