<noscript> is only activated if JS is actively disabled. None of these failure modes will trigger it. You can use onerror="foo" on <script> tags though EDIT: A

<noscript> is only activated if JS is actively disabled. None of these failure modes will trigger it. You can use onerror="foo" on <script> tags though EDIT: Actually the data saver thing is called "noscript" so you'd hope it'd activate the tags. Network failures though will trigger onerror and leave the rest up to you