HTML if statement to load local JS/CSS upon CDN failure

Question

When loading a CSS/JS file from CDN or any external server, it is possible (even with low probability) to miss the file due to external failure. In this case, the html page will

Answer 1

For Javascript, you can listen for the onload and onerror events when building a dynamic script. However, in those same pages, it shows otherwise for CSS.

The only reliable way to dynamically load CSS is to do it via AJAX. You could load the styles via dynamic link tags but without those events, you won't know if they have been loaded at all. You could poll for the styles, but it's hackish IMO.

Another way to do it is make the server read those CDN files. If they're good, print the urls for the links. But if those links are dead, make it print the local urls instead. This would be more reliable, and offloads your logic to the server. This assumes that you have access to the server.

Or better, use the local versions in the first place! With good caching, bandwidth won't be an issue