Certain firewalls / proxies strip CORS Allow-Origin headers (example); measure how many users are affected by this:
- create a test script which sets some global variable
- before DOM ready, add a script tag pointing to that script with crossorigin="anonymous"
- after DOM ready, check the global variable; report back if not set.
Results: CORS-enabled loading of script files causes loading failure in about 0.1%-0.15% of the sampled pageviews.