When a Content Policy blocks a script, the error from the script is not handled with an error event that propogates to the client (and is hence displayed to the developer in the Error Console or Web Console). I discovered this while trying to catch a script load error with the onerror event when a the mixed content Content Policy blocked the script. I believe the error should be handled in CheckContentPolicy in nsScriptLoader.cpp: mxr.mozilla.org/mozilla-central/source/content/base/src/nsScriptLoader.cpp#212

So this is basically about us firing onerror on the script if the script is blocked by CheckLoadURI or content policy? Jonas, any objections? In particular, would this let sites detect when ad-blockers are in use in ways they can't right now?

(In reply to Boris Zbarsky (:bz) from comment #1) > Jonas, any objections? In particular, would this let sites detect when > ad-blockers are in use in ways they can't right now? I doubt that's possible: it's trivial to add a side-effect to a script and check whether that side-effect happened in a later script in the page.

I agree. It will always be possible to detect that a script is blocked, so being explicit about it seems ok.

Comment on attachment 661587 [details] [diff] [review] Fire error events on <script> elements which completely fail to start the load. >+ test_bug789856.html \ This file is missing.

Good catch!