User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 11_1_0) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/88.0.4324.96 Safari/537.36

Steps to reproduce:

Evaluate new MediaSource().onsourceclose and new MediaSource().onsourceclosed. Or try using the onsourceclose (sans d) event handler attribute to handle the "sourceclose" event.

Actual results:

The supported event handler is "onsourceclosed" with an extra "d". However, the event that is actually fired doesn't have the extra "d", so this event handler attribute doesn't do anything:
https://github.com/mozilla/gecko-dev/blob/b9384b091e901b3283ce24b6610e80699d79fd06/dom/media/mediasource/MediaSource.cpp#L536

Expected results:

It needs to be renamed to drop the trailing "d"

There's a Firefox-only failure for this in WPT:
https://wpt.fyi/results/media-source/idlharness.window.html?label=master&label=experimental&product=chrome&product=firefox&product=safari&aligned

And here's the line in your Web IDL that needs updating: https://github.com/mozilla/gecko-dev/blob/b9384b091e901b3283ce24b6610e80699d79fd06/dom/webidl/MediaSource.webidl#L37

Bugbug thinks this bug should belong to this component, but please revert this change in case of error.

I can see the failure in http://wpt.live/media-source/idlharness.window.html. Looks like it should be relatively straightforward to fix. Thanks for the report!

https://hg.mozilla.org/mozilla-central/rev/2651c988ef75