In theory mask is not needed since webms should support alpha. But my animations still have black background without masks ingame even with alpha enabled, so I suppose renpy itself wants the mask webm no matter what.
That's sort of a half truth... webm's do "kind of" support alpha channel but it's a bit of a late-addon hack and while it was supposed to be safe and not mess with the format, a lot of player break when such a video is played. There is a support for it on Chrome browsers, but it doesn't seem to be gaining much attention from developers.
When I was looking into it, what they did in Ren'Py is actually considered a very decent solution. Although, to be perfectly honest, ,gif support might have been more welcome, even if it is an outdated format... I mean, they are trying to kill (unrelated) Flash for like a decade and still sh!tloads of games/players/content are made with it, when something got into the mainstream, getting it out to make a way for something new is not easy.