Currently, even when a WEBM or MP3 file is embedded behind a spoiler, it automatically begins playing making it impossible to embed video files containing sound (with the exception of SWFs for some reason) without breaking the rules. (Note: Preventing autoplay behind spoilers does work for Chrome, but not for any other browser).
As you can see, it automatically plays whether or not the spoiler is opened.
I'm not sure how to solve this issue, but it would be useful for spoilers to stop video files from autoplaying, or even better, find a way to stop video files from autoplaying altogether (using autoplay=0
doesn't work).