Funny thing, I tried on android and all scenes were fine. But on PC, pretty much all animation shows black screen... And I was hoping I wouldn't have to put more games onto my already-low-on-storage phone...
I just tested the game for the mute option to be sure and it actually doesn't seem to be an issue for this newer version of Ren'Py any more, videos are still playing without sound enabled. So you can forget about this possible issue.
What OS are you running on that PC? It's a critical question, not just curiosity.
Windows 10 and 11 have the webm decoder installed by default and Ren'Py should be able to play webm videos without any issues.
Any older Windows and you need to install the webm decoder by yourself:
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On Windows 10 and 11, if the videos aren't playing, the codec package may also be uninstalled or corrupted.
I think, in Windows 10 and 11 webm is likely installed with the "Web Media Extensions" package from the MS Store
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But I'm not completely sure about this. It was installed on my Windows 11 by default and its description says that it adds support for Ogg, Vorbis and Theora encoded content, but this article sheds a little more light on it:
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If you are using Windows 10 or 11, you can just open Microsoft Store and make sure that "Web Media Extensions" package is installed. I'd also recommend installing WebP image extension, HEIF image extension, AV1 Video extension and MPEG2 video extension in case they aren't installed yet. They are all free, no cost.
Old Macs don't support webm, either. But the codec can be installed via a third party repository for Macs. I can look it up in case you have a Mac. I've recommended it to a couple of old Mac users before and it worked just fine.
On Linux it may just be a matter of installing a missing package. Depends on distribution.
Android devices that aren't too old to run the OS and games on it are obviously already new enough systems to have webm support installed by default.